<?xml version="1.0" encoding="utf-8"?>
<!--XBRL document created with Toppan Merrill Bridge  10.7.0.1 -->
<!--Based on XBRL 2.1-->
<!--Created on: 8/5/2024 6:31:36 PM-->
<!--Modified on: 8/5/2024 6:31:36 PM-->
<link:linkbase xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.xbrl.org/2003/linkbase http://www.xbrl.org/2003/xbrl-linkbase-2003-12-31.xsd" xmlns:link="http://www.xbrl.org/2003/linkbase" xmlns:xbrldt="http://xbrl.org/2005/xbrldt" xmlns:xbrli="http://www.xbrl.org/2003/instance" xmlns:xlink="http://www.w3.org/1999/xlink">
  <roleRef xlink:type="simple" xlink:href="aciu-20240630.xsd#StatementConsolidatedStatementsOfChangesInEquity" roleURI="http://acimume.com/role/StatementConsolidatedStatementsOfChangesInEquity" xmlns="http://www.xbrl.org/2003/linkbase" />
  <roleRef xlink:type="simple" xlink:href="aciu-20240630.xsd#DisclosureContractRevenuesAndOtherOperatingIncomeDetails" roleURI="http://acimume.com/role/DisclosureContractRevenuesAndOtherOperatingIncomeDetails" xmlns="http://www.xbrl.org/2003/linkbase" />
  <roleRef xlink:type="simple" xlink:href="aciu-20240630.xsd#DisclosureContractRevenuesAndOtherOperatingIncomeAntiAbetaActiveImmunotherapyInAd2024AgreementTakedaPharmaceuticalsUsaIncDetails" roleURI="http://acimume.com/role/DisclosureContractRevenuesAndOtherOperatingIncomeAntiAbetaActiveImmunotherapyInAd2024AgreementTakedaPharmaceuticalsUsaIncDetails" xmlns="http://www.xbrl.org/2003/linkbase" />
  <roleRef xlink:type="simple" xlink:href="aciu-20240630.xsd#DisclosureContractRevenuesAndOtherOperatingIncomeGrantsFromMichaelJFoxFoundationDetails" roleURI="http://acimume.com/role/DisclosureContractRevenuesAndOtherOperatingIncomeGrantsFromMichaelJFoxFoundationDetails" xmlns="http://www.xbrl.org/2003/linkbase" />
  <roleRef xlink:type="simple" xlink:href="aciu-20240630.xsd#DisclosureLossPerShareAntidilutiveSecuritiesDetails" roleURI="http://acimume.com/role/DisclosureLossPerShareAntidilutiveSecuritiesDetails" xmlns="http://www.xbrl.org/2003/linkbase" />
  <roleRef xlink:type="simple" xlink:href="aciu-20240630.xsd#DisclosurePropertyPlantAndEquipmentDetails" roleURI="http://acimume.com/role/DisclosurePropertyPlantAndEquipmentDetails" xmlns="http://www.xbrl.org/2003/linkbase" />
  <roleRef xlink:type="simple" xlink:href="aciu-20240630.xsd#DisclosureRightOfUseAssetsLongTermFinancialAssetsAndLeaseLiabilitiesDetails" roleURI="http://acimume.com/role/DisclosureRightOfUseAssetsLongTermFinancialAssetsAndLeaseLiabilitiesDetails" xmlns="http://www.xbrl.org/2003/linkbase" />
  <roleRef xlink:type="simple" xlink:href="aciu-20240630.xsd#DisclosureRightOfUseAssetsLongTermFinancialAssetsAndLeaseLiabilitiesContractualUndiscountedCashFlowsForLeaseObligationsDetails" roleURI="http://acimume.com/role/DisclosureRightOfUseAssetsLongTermFinancialAssetsAndLeaseLiabilitiesContractualUndiscountedCashFlowsForLeaseObligationsDetails" xmlns="http://www.xbrl.org/2003/linkbase" />
  <roleRef xlink:type="simple" xlink:href="aciu-20240630.xsd#DisclosureIntangibleAssetsDetails" roleURI="http://acimume.com/role/DisclosureIntangibleAssetsDetails" xmlns="http://www.xbrl.org/2003/linkbase" />
  <roleRef xlink:type="simple" xlink:href="aciu-20240630.xsd#DisclosureShareCapitalAndTreasurySharesDetails" roleURI="http://acimume.com/role/DisclosureShareCapitalAndTreasurySharesDetails" xmlns="http://www.xbrl.org/2003/linkbase" />
  <link:roleRef roleURI="http://acimume.com/role/DisclosureStandardAndCustomAxisDomainDefaults" xlink:type="simple" xlink:href="aciu-20240630.xsd#DisclosureStandardAndCustomAxisDomainDefaults" />
  <link:roleRef roleURI="http://acimume.com/role/DisclosureLossPerShareDetails" xlink:type="simple" xlink:href="aciu-20240630.xsd#DisclosureLossPerShareDetails" />
  <link:arcroleRef arcroleURI="http://www.esma.europa.eu/xbrl/esef/arcrole/wider-narrower" xlink:type="simple" xlink:href="http://www.xbrl.org/lrr/arcrole/esma-arcrole-2018-11-21.xsd#wider-narrower" />
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/all" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#all" />
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-default" />
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-domain" />
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/domain-member" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#domain-member" />
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#hypercube-dimension" />
  <link:definitionLink xlink:type="extended" xlink:role="http://acimume.com/role/DisclosureLossPerShareDetails" />
  <link:definitionLink xlink:type="extended" xlink:role="http://acimume.com/role/DisclosureStandardAndCustomAxisDomainDefaults">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2023/dei-2023.xsd#dei_EntityAddressesAddressTypeAxis" xlink:label="EntityAddressesAddressTypeAxis" xlink:title="EntityAddressesAddressTypeAxis" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2023/dei-2023.xsd#dei_AddressTypeDomain" xlink:label="AddressTypeDomain" xlink:title="AddressTypeDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="EntityAddressesAddressTypeAxis" xlink:to="AddressTypeDomain" xlink:title="definition: EntityAddressesAddressTypeAxis to AddressTypeDomain" use="optional" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_ClassesOfPropertyPlantAndEquipmentAxis" xlink:label="ClassesOfPropertyPlantAndEquipmentAxis" xlink:title="ClassesOfPropertyPlantAndEquipmentAxis" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_PropertyPlantAndEquipmentMember" xlink:label="PropertyPlantAndEquipmentMember" xlink:title="PropertyPlantAndEquipmentMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="ClassesOfPropertyPlantAndEquipmentAxis" xlink:to="PropertyPlantAndEquipmentMember" xlink:title="definition: ClassesOfPropertyPlantAndEquipmentAxis to PropertyPlantAndEquipmentMember" use="optional" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_ClassesOfAssetsAxis" xlink:label="ClassesOfAssetsAxis" xlink:title="ClassesOfAssetsAxis" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_ClassesOfAssetsMember" xlink:label="ClassesOfAssetsMember" xlink:title="ClassesOfAssetsMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="ClassesOfAssetsAxis" xlink:to="ClassesOfAssetsMember" xlink:title="definition: ClassesOfAssetsAxis to ClassesOfAssetsMember" use="optional" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_CarryingAmountAccumulatedDepreciationAmortisationAndImpairmentAndGrossCarryingAmountAxis" xlink:label="CarryingAmountAccumulatedDepreciationAmortisationAndImpairmentAndGrossCarryingAmountAxis" xlink:title="CarryingAmountAccumulatedDepreciationAmortisationAndImpairmentAndGrossCarryingAmountAxis" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_CarryingAmountMember" xlink:label="CarryingAmountMember" xlink:title="CarryingAmountMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="CarryingAmountAccumulatedDepreciationAmortisationAndImpairmentAndGrossCarryingAmountAxis" xlink:to="CarryingAmountMember" xlink:title="definition: CarryingAmountAccumulatedDepreciationAmortisationAndImpairmentAndGrossCarryingAmountAxis to CarryingAmountMember" use="optional" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_MaturityAxis" xlink:label="MaturityAxis" xlink:title="MaturityAxis" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_AggregatedTimeBandsMember" xlink:label="AggregatedTimeBandsMember" xlink:title="AggregatedTimeBandsMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="MaturityAxis" xlink:to="AggregatedTimeBandsMember" xlink:title="definition: MaturityAxis to AggregatedTimeBandsMember" use="optional" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_ClassesOfIntangibleAssetsOtherThanGoodwillAxis" xlink:label="ClassesOfIntangibleAssetsOtherThanGoodwillAxis" xlink:title="ClassesOfIntangibleAssetsOtherThanGoodwillAxis" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_IntangibleAssetsOtherThanGoodwillMember" xlink:label="IntangibleAssetsOtherThanGoodwillMember" xlink:title="IntangibleAssetsOtherThanGoodwillMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="ClassesOfIntangibleAssetsOtherThanGoodwillAxis" xlink:to="IntangibleAssetsOtherThanGoodwillMember" xlink:title="definition: ClassesOfIntangibleAssetsOtherThanGoodwillAxis to IntangibleAssetsOtherThanGoodwillMember" use="optional" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_UnobservableInputsAxis" xlink:label="UnobservableInputsAxis" xlink:title="UnobservableInputsAxis" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_UnobservableInputsMember" xlink:label="UnobservableInputsMember" xlink:title="UnobservableInputsMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="UnobservableInputsAxis" xlink:to="UnobservableInputsMember" xlink:title="definition: UnobservableInputsAxis to UnobservableInputsMember" use="optional" order="1" />
    <link:loc xlink:type="locator" xlink:href="aciu-20240630.xsd#aciu_AssetAcquisitionsAxis" xlink:label="AssetAcquisitionsAxis" xlink:title="AssetAcquisitionsAxis" />
    <link:loc xlink:type="locator" xlink:href="aciu-20240630.xsd#aciu_SignificantAssetAcquisitionsTotalDomain" xlink:label="SignificantAssetAcquisitionsTotalDomain" xlink:title="SignificantAssetAcquisitionsTotalDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="AssetAcquisitionsAxis" xlink:to="SignificantAssetAcquisitionsTotalDomain" xlink:title="definition: AssetAcquisitionsAxis to SignificantAssetAcquisitionsTotalDomain" use="optional" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_CurrencyAxis" xlink:label="CurrencyAxis" xlink:title="CurrencyAxis" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/currency/2023/currency-2023.xsd#currency_AllCurrenciesDomain" xlink:label="AllCurrenciesDomain" xlink:title="AllCurrenciesDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="CurrencyAxis" xlink:to="AllCurrenciesDomain" xlink:title="definition: CurrencyAxis to AllCurrenciesDomain" use="optional" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_TypesOfContractsAxis" xlink:label="TypesOfContractsAxis" xlink:title="TypesOfContractsAxis" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_TypesOfContractsMember" xlink:label="TypesOfContractsMember" xlink:title="TypesOfContractsMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="TypesOfContractsAxis" xlink:to="TypesOfContractsMember" xlink:title="definition: TypesOfContractsAxis to TypesOfContractsMember" use="optional" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_ClassesOfShareCapitalAxis" xlink:label="ClassesOfShareCapitalAxis" xlink:title="ClassesOfShareCapitalAxis" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_ClassesOfShareCapitalMember" xlink:label="ClassesOfShareCapitalMember" xlink:title="ClassesOfShareCapitalMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="ClassesOfShareCapitalAxis" xlink:to="ClassesOfShareCapitalMember" xlink:title="definition: ClassesOfShareCapitalAxis to ClassesOfShareCapitalMember" use="optional" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_RangeAxis" xlink:label="RangeAxis" xlink:title="RangeAxis" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_RangesMember" xlink:label="RangesMember" xlink:title="RangesMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="RangeAxis" xlink:to="RangesMember" xlink:title="definition: RangeAxis to RangesMember" use="optional" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_StatementScenarioAxis" xlink:label="StatementScenarioAxis" xlink:title="StatementScenarioAxis" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_ScenarioUnspecifiedDomain" xlink:label="ScenarioUnspecifiedDomain" xlink:title="ScenarioUnspecifiedDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="StatementScenarioAxis" xlink:to="ScenarioUnspecifiedDomain" xlink:title="definition: StatementScenarioAxis to ScenarioUnspecifiedDomain" use="optional" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_CounterpartiesAxis" xlink:label="CounterpartiesAxis" xlink:title="CounterpartiesAxis" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_CounterpartiesMember" xlink:label="CounterpartiesMember" xlink:title="CounterpartiesMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="CounterpartiesAxis" xlink:to="CounterpartiesMember" xlink:title="definition: CounterpartiesAxis to CounterpartiesMember" use="optional" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_ComponentsOfEquityAxis" xlink:label="ComponentsOfEquityAxis" xlink:title="ComponentsOfEquityAxis" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_EquityMember" xlink:label="EquityMember" xlink:title="EquityMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="ComponentsOfEquityAxis" xlink:to="EquityMember" xlink:title="definition: ComponentsOfEquityAxis to EquityMember" use="optional" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_ProductsAndServicesAxis" xlink:label="ProductsAndServicesAxis" xlink:title="ProductsAndServicesAxis" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_ProductsAndServicesMember" xlink:label="ProductsAndServicesMember" xlink:title="ProductsAndServicesMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="ProductsAndServicesAxis" xlink:to="ProductsAndServicesMember" xlink:title="definition: ProductsAndServicesAxis to ProductsAndServicesMember" use="optional" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_PerformanceObligationsAxis" xlink:label="PerformanceObligationsAxis" xlink:title="PerformanceObligationsAxis" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_PerformanceObligationsMember" xlink:label="PerformanceObligationsMember" xlink:title="PerformanceObligationsMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="PerformanceObligationsAxis" xlink:to="PerformanceObligationsMember" xlink:title="definition: PerformanceObligationsAxis to PerformanceObligationsMember" use="optional" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2023/dei-2023.xsd#dei_LegalEntityAxis" xlink:label="LegalEntityAxis" xlink:title="LegalEntityAxis" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2023/dei-2023.xsd#dei_EntityDomain" xlink:label="EntityDomain" xlink:title="EntityDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="LegalEntityAxis" xlink:to="EntityDomain" xlink:title="definition: LegalEntityAxis to EntityDomain" use="optional" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_BorrowingsByNameAxis" xlink:label="BorrowingsByNameAxis" xlink:title="BorrowingsByNameAxis" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_BorrowingsByNameMember" xlink:label="BorrowingsByNameMember" xlink:title="BorrowingsByNameMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="BorrowingsByNameAxis" xlink:to="BorrowingsByNameMember" xlink:title="definition: BorrowingsByNameAxis to BorrowingsByNameMember" use="optional" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_AttributionOfExpensesByNatureToTheirFunctionAxis" xlink:label="AttributionOfExpensesByNatureToTheirFunctionAxis" xlink:title="AttributionOfExpensesByNatureToTheirFunctionAxis" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_LineItemsByFunctionMember" xlink:label="LineItemsByFunctionMember" xlink:title="LineItemsByFunctionMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="AttributionOfExpensesByNatureToTheirFunctionAxis" xlink:to="LineItemsByFunctionMember" xlink:title="definition: AttributionOfExpensesByNatureToTheirFunctionAxis to LineItemsByFunctionMember" use="optional" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_TemporaryDifferenceUnusedTaxLossesAndUnusedTaxCreditsAxis" xlink:label="TemporaryDifferenceUnusedTaxLossesAndUnusedTaxCreditsAxis" xlink:title="TemporaryDifferenceUnusedTaxLossesAndUnusedTaxCreditsAxis" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_TemporaryDifferenceUnusedTaxLossesAndUnusedTaxCreditsMember" xlink:label="TemporaryDifferenceUnusedTaxLossesAndUnusedTaxCreditsMember" xlink:title="TemporaryDifferenceUnusedTaxLossesAndUnusedTaxCreditsMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="TemporaryDifferenceUnusedTaxLossesAndUnusedTaxCreditsAxis" xlink:to="TemporaryDifferenceUnusedTaxLossesAndUnusedTaxCreditsMember" xlink:title="definition: TemporaryDifferenceUnusedTaxLossesAndUnusedTaxCreditsAxis to TemporaryDifferenceUnusedTaxLossesAndUnusedTaxCreditsMember" use="optional" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_GeographicalAreasAxis" xlink:label="GeographicalAreasAxis" xlink:title="GeographicalAreasAxis" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_GeographicalAreasMember" xlink:label="GeographicalAreasMember" xlink:title="GeographicalAreasMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="GeographicalAreasAxis" xlink:to="GeographicalAreasMember" xlink:title="definition: GeographicalAreasAxis to GeographicalAreasMember" use="optional" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_DefinedBenefitPlansAxis" xlink:label="DefinedBenefitPlansAxis" xlink:title="DefinedBenefitPlansAxis" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_DefinedBenefitPlansMember" xlink:label="DefinedBenefitPlansMember" xlink:title="DefinedBenefitPlansMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="DefinedBenefitPlansAxis" xlink:to="DefinedBenefitPlansMember" xlink:title="definition: DefinedBenefitPlansAxis to DefinedBenefitPlansMember" use="optional" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_NetDefinedBenefitLiabilityAssetAxis" xlink:label="NetDefinedBenefitLiabilityAssetAxis" xlink:title="NetDefinedBenefitLiabilityAssetAxis" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_NetDefinedBenefitLiabilityAssetMember" xlink:label="NetDefinedBenefitLiabilityAssetMember" xlink:title="NetDefinedBenefitLiabilityAssetMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="NetDefinedBenefitLiabilityAssetAxis" xlink:to="NetDefinedBenefitLiabilityAssetMember" xlink:title="definition: NetDefinedBenefitLiabilityAssetAxis to NetDefinedBenefitLiabilityAssetMember" use="optional" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_ActuarialAssumptionsAxis" xlink:label="ActuarialAssumptionsAxis" xlink:title="ActuarialAssumptionsAxis" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_ActuarialAssumptionsMember" xlink:label="ActuarialAssumptionsMember" xlink:title="ActuarialAssumptionsMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="ActuarialAssumptionsAxis" xlink:to="ActuarialAssumptionsMember" xlink:title="definition: ActuarialAssumptionsAxis to ActuarialAssumptionsMember" use="optional" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_TypesOfRisksAxis" xlink:label="TypesOfRisksAxis" xlink:title="TypesOfRisksAxis" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_TypesOfRisksMember" xlink:label="TypesOfRisksMember" xlink:title="TypesOfRisksMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="TypesOfRisksAxis" xlink:to="TypesOfRisksMember" xlink:title="definition: TypesOfRisksAxis to TypesOfRisksMember" use="optional" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_RangesOfExercisePricesForOutstandingShareOptionsAxis" xlink:label="RangesOfExercisePricesForOutstandingShareOptionsAxis" xlink:title="RangesOfExercisePricesForOutstandingShareOptionsAxis" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_RangesOfExercisePricesForOutstandingShareOptionsMember" xlink:label="RangesOfExercisePricesForOutstandingShareOptionsMember" xlink:title="RangesOfExercisePricesForOutstandingShareOptionsMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="RangesOfExercisePricesForOutstandingShareOptionsAxis" xlink:to="RangesOfExercisePricesForOutstandingShareOptionsMember" xlink:title="definition: RangesOfExercisePricesForOutstandingShareOptionsAxis to RangesOfExercisePricesForOutstandingShareOptionsMember" use="optional" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_CategoriesOfFinancialAssetsAxis" xlink:label="CategoriesOfFinancialAssetsAxis" xlink:title="CategoriesOfFinancialAssetsAxis" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_FinancialAssetsCategoryMember" xlink:label="FinancialAssetsCategoryMember" xlink:title="FinancialAssetsCategoryMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="CategoriesOfFinancialAssetsAxis" xlink:to="FinancialAssetsCategoryMember" xlink:title="definition: CategoriesOfFinancialAssetsAxis to FinancialAssetsCategoryMember" use="optional" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_CategoriesOfFinancialLiabilitiesAxis" xlink:label="CategoriesOfFinancialLiabilitiesAxis" xlink:title="CategoriesOfFinancialLiabilitiesAxis" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_FinancialLiabilitiesCategoryMember" xlink:label="FinancialLiabilitiesCategoryMember" xlink:title="FinancialLiabilitiesCategoryMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="CategoriesOfFinancialLiabilitiesAxis" xlink:to="FinancialLiabilitiesCategoryMember" xlink:title="definition: CategoriesOfFinancialLiabilitiesAxis to FinancialLiabilitiesCategoryMember" use="optional" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_ClassesOfFinancialAssetsAxis" xlink:label="ClassesOfFinancialAssetsAxis" xlink:title="ClassesOfFinancialAssetsAxis" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_FinancialAssetsMember" xlink:label="FinancialAssetsMember" xlink:title="FinancialAssetsMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="ClassesOfFinancialAssetsAxis" xlink:to="FinancialAssetsMember" xlink:title="definition: ClassesOfFinancialAssetsAxis to FinancialAssetsMember" use="optional" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_ClassesOfFinancialLiabilitiesAxis" xlink:label="ClassesOfFinancialLiabilitiesAxis" xlink:title="ClassesOfFinancialLiabilitiesAxis" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_FinancialLiabilitiesMember" xlink:label="FinancialLiabilitiesMember" xlink:title="FinancialLiabilitiesMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="ClassesOfFinancialLiabilitiesAxis" xlink:to="FinancialLiabilitiesMember" xlink:title="definition: ClassesOfFinancialLiabilitiesAxis to FinancialLiabilitiesMember" use="optional" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_BusinessCombinationsAxis" xlink:label="BusinessCombinationsAxis" xlink:title="BusinessCombinationsAxis" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_EntitysTotalForBusinessCombinationsMember" xlink:label="EntitysTotalForBusinessCombinationsMember" xlink:title="EntitysTotalForBusinessCombinationsMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="BusinessCombinationsAxis" xlink:to="EntitysTotalForBusinessCombinationsMember" xlink:title="definition: BusinessCombinationsAxis to EntitysTotalForBusinessCombinationsMember" use="optional" order="1" />
    <link:loc xlink:type="locator" xlink:href="aciu-20240630.xsd#aciu_SignificantAssetAcquisitionsAxis" xlink:label="SignificantAssetAcquisitionsAxis" xlink:title="SignificantAssetAcquisitionsAxis" />
    <link:loc xlink:type="locator" xlink:href="aciu-20240630.xsd#aciu_AllSignificantAssetAcquisitionsDomain" xlink:label="AllSignificantAssetAcquisitionsDomain" xlink:title="AllSignificantAssetAcquisitionsDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="SignificantAssetAcquisitionsAxis" xlink:to="AllSignificantAssetAcquisitionsDomain" xlink:title="definition: SignificantAssetAcquisitionsAxis to AllSignificantAssetAcquisitionsDomain" use="optional" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_ClassesOfOrdinarySharesAxis" xlink:label="ClassesOfOrdinarySharesAxis" xlink:title="ClassesOfOrdinarySharesAxis" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_OrdinarySharesMember" xlink:label="OrdinarySharesMember" xlink:title="OrdinarySharesMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="ClassesOfOrdinarySharesAxis" xlink:to="OrdinarySharesMember" xlink:title="definition: ClassesOfOrdinarySharesAxis to OrdinarySharesMember" use="optional" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_TypesOfSharebasedPaymentArrangementsAxis" xlink:label="TypesOfSharebasedPaymentArrangementsAxis" xlink:title="TypesOfSharebasedPaymentArrangementsAxis" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_SharebasedPaymentArrangementsMember" xlink:label="SharebasedPaymentArrangementsMember" xlink:title="SharebasedPaymentArrangementsMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="TypesOfSharebasedPaymentArrangementsAxis" xlink:to="SharebasedPaymentArrangementsMember" xlink:title="definition: TypesOfSharebasedPaymentArrangementsAxis to SharebasedPaymentArrangementsMember" use="optional" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_CategoriesOfRelatedPartiesAxis" xlink:label="CategoriesOfRelatedPartiesAxis" xlink:title="CategoriesOfRelatedPartiesAxis" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_EntitysTotalForRelatedPartiesMember" xlink:label="EntitysTotalForRelatedPartiesMember" xlink:title="EntitysTotalForRelatedPartiesMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="CategoriesOfRelatedPartiesAxis" xlink:to="EntitysTotalForRelatedPartiesMember" xlink:title="definition: CategoriesOfRelatedPartiesAxis to EntitysTotalForRelatedPartiesMember" use="optional" order="1" />
    <link:loc xlink:type="locator" xlink:href="aciu-20240630.xsd#aciu_IfrsSubsequentEventTypeAxis" xlink:label="aciu_IfrsSubsequentEventTypeAxis_1" xlink:title="aciu_IfrsSubsequentEventTypeAxis_1" />
    <link:loc xlink:type="locator" xlink:href="aciu-20240630.xsd#aciu_IfrsSubsequentEventTypeDomain" xlink:label="aciu_IfrsSubsequentEventTypeDomain_1" xlink:title="aciu_IfrsSubsequentEventTypeDomain_1" />
    <link:definitionArc xlink:type="arc" xlink:from="aciu_IfrsSubsequentEventTypeAxis_1" xlink:to="aciu_IfrsSubsequentEventTypeDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:aciu_IfrsSubsequentEventTypeAxis_1 To aciu_IfrsSubsequentEventTypeDomain_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_NonadjustingEventsAfterReportingPeriodAxis" xlink:label="ifrs-full_NonadjustingEventsAfterReportingPeriodAxis_1" xlink:title="ifrs-full_NonadjustingEventsAfterReportingPeriodAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_NonadjustingEventsMember" xlink:label="ifrs-full_NonadjustingEventsMember_1" xlink:title="ifrs-full_NonadjustingEventsMember_1" />
    <link:definitionArc xlink:type="arc" xlink:from="ifrs-full_NonadjustingEventsAfterReportingPeriodAxis_1" xlink:to="ifrs-full_NonadjustingEventsMember_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:ifrs-full_NonadjustingEventsAfterReportingPeriodAxis_1 To ifrs-full_NonadjustingEventsMember_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_TypesOfAntidilutiveInstrumentsAxis" xlink:label="ifrs-full_TypesOfAntidilutiveInstrumentsAxis_1" xlink:title="ifrs-full_TypesOfAntidilutiveInstrumentsAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_AntidilutiveInstrumentsMember" xlink:label="ifrs-full_AntidilutiveInstrumentsMember_1" xlink:title="ifrs-full_AntidilutiveInstrumentsMember_1" />
    <link:definitionArc xlink:type="arc" xlink:from="ifrs-full_TypesOfAntidilutiveInstrumentsAxis_1" xlink:to="ifrs-full_AntidilutiveInstrumentsMember_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:ifrs-full_TypesOfAntidilutiveInstrumentsAxis_1 To ifrs-full_AntidilutiveInstrumentsMember_1" order="1" />
  </link:definitionLink>
  <definitionLink xlink:role="http://acimume.com/role/StatementConsolidatedStatementsOfChangesInEquity" xlink:type="extended" xlink:title="00400 - Statement - Consolidated Statements of Changes in Equity" xmlns="http://www.xbrl.org/2003/linkbase">
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_StatementOfChangesInEquityTable" xlink:label="ifrs-full_StatementOfChangesInEquityTable" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_ComponentsOfEquityAxis" xlink:label="ifrs-full_ComponentsOfEquityAxis" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="1" xlink:from="ifrs-full_StatementOfChangesInEquityTable" xlink:to="ifrs-full_ComponentsOfEquityAxis" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_EquityMember" xlink:label="ifrs-full_EquityMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="ifrs-full_ComponentsOfEquityAxis" xlink:to="ifrs-full_EquityMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_IssuedCapitalMember" xlink:label="ifrs-full_IssuedCapitalMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="ifrs-full_EquityMember" xlink:to="ifrs-full_IssuedCapitalMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_SharePremiumMember" xlink:label="ifrs-full_SharePremiumMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="ifrs-full_EquityMember" xlink:to="ifrs-full_SharePremiumMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_TreasurySharesMember" xlink:label="ifrs-full_TreasurySharesMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="ifrs-full_EquityMember" xlink:to="ifrs-full_TreasurySharesMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_RetainedEarningsMember" xlink:label="ifrs-full_RetainedEarningsMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="ifrs-full_EquityMember" xlink:to="ifrs-full_RetainedEarningsMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_ReserveOfExchangeDifferencesOnTranslationMember" xlink:label="ifrs-full_ReserveOfExchangeDifferencesOnTranslationMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="5" xlink:from="ifrs-full_EquityMember" xlink:to="ifrs-full_ReserveOfExchangeDifferencesOnTranslationMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_StatementOfChangesInEquityLineItems" xlink:label="ifrs-full_StatementOfChangesInEquityLineItems" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" order="1" xlink:from="ifrs-full_StatementOfChangesInEquityLineItems" xlink:to="ifrs-full_StatementOfChangesInEquityTable" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_Equity" xlink:label="ifrs-full_Equity" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="ifrs-full_StatementOfChangesInEquityLineItems" xlink:to="ifrs-full_Equity" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_ProfitLoss" xlink:label="ifrs-full_ProfitLoss" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="ifrs-full_StatementOfChangesInEquityLineItems" xlink:to="ifrs-full_ProfitLoss" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_OtherComprehensiveIncome" xlink:label="ifrs-full_OtherComprehensiveIncome" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="ifrs-full_StatementOfChangesInEquityLineItems" xlink:to="ifrs-full_OtherComprehensiveIncome" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_ComprehensiveIncome" xlink:label="ifrs-full_ComprehensiveIncome" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="5" xlink:from="ifrs-full_StatementOfChangesInEquityLineItems" xlink:to="ifrs-full_ComprehensiveIncome" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_IncreaseDecreaseThroughSharebasedPaymentTransactions" xlink:label="ifrs-full_IncreaseDecreaseThroughSharebasedPaymentTransactions" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="6" xlink:from="ifrs-full_StatementOfChangesInEquityLineItems" xlink:to="ifrs-full_IncreaseDecreaseThroughSharebasedPaymentTransactions" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="aciu-20240630.xsd#aciu_SaleOrIssueOfTreasurySharesNetOfUnderwritingFeesAndTransactionCosts" xlink:label="aciu_SaleOrIssueOfTreasurySharesNetOfUnderwritingFeesAndTransactionCosts" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="7" xlink:from="ifrs-full_StatementOfChangesInEquityLineItems" xlink:to="aciu_SaleOrIssueOfTreasurySharesNetOfUnderwritingFeesAndTransactionCosts" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="aciu-20240630.xsd#aciu_IncreaseDecreaseThroughIssueOfSharesToBeHeldAsTreasurySharesEquity" xlink:label="aciu_IncreaseDecreaseThroughIssueOfSharesToBeHeldAsTreasurySharesEquity" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="8" xlink:from="ifrs-full_StatementOfChangesInEquityLineItems" xlink:to="aciu_IncreaseDecreaseThroughIssueOfSharesToBeHeldAsTreasurySharesEquity" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_NumberOfSharesIssuedAbstract" xlink:label="ifrs-full_NumberOfSharesIssuedAbstract" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="9" xlink:from="ifrs-full_StatementOfChangesInEquityLineItems" xlink:to="ifrs-full_NumberOfSharesIssuedAbstract" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="aciu-20240630.xsd#aciu_IssueOfEquityRestrictedShareAwards" xlink:label="aciu_IssueOfEquityRestrictedShareAwards" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="ifrs-full_NumberOfSharesIssuedAbstract" xlink:to="aciu_IssueOfEquityRestrictedShareAwards" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_IncreaseDecreaseThroughExerciseOfOptions" xlink:label="ifrs-full_IncreaseDecreaseThroughExerciseOfOptions" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="ifrs-full_NumberOfSharesIssuedAbstract" xlink:to="ifrs-full_IncreaseDecreaseThroughExerciseOfOptions" priority="1" use="optional" />
  </definitionLink>
  <definitionLink xlink:role="http://acimume.com/role/DisclosureContractRevenuesAndOtherOperatingIncomeDetails" xlink:type="extended" xlink:title="40301 - Disclosure - Contract revenues and other operating income (Details)" xmlns="http://www.xbrl.org/2003/linkbase">
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_DisclosureOfDisaggregationOfRevenueFromContractsWithCustomersTable" xlink:label="ifrs-full_DisclosureOfDisaggregationOfRevenueFromContractsWithCustomersTable" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_TypesOfContractsAxis" xlink:label="ifrs-full_TypesOfContractsAxis" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="1" xlink:from="ifrs-full_DisclosureOfDisaggregationOfRevenueFromContractsWithCustomersTable" xlink:to="ifrs-full_TypesOfContractsAxis" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_TypesOfContractsMember" xlink:label="ifrs-full_TypesOfContractsMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="ifrs-full_TypesOfContractsAxis" xlink:to="ifrs-full_TypesOfContractsMember" use="optional" />
    <loc xlink:type="locator" xlink:href="aciu-20240630.xsd#aciu_AntiAbetaActiveImmunotherapyInAd2024AgreementTakedaPharmaceuticalsUsaInc.Member" xlink:label="aciu_AntiAbetaActiveImmunotherapyInAd2024AgreementTakedaPharmaceuticalsUsaInc.Member" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="ifrs-full_TypesOfContractsMember" xlink:to="aciu_AntiAbetaActiveImmunotherapyInAd2024AgreementTakedaPharmaceuticalsUsaInc.Member" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_DisclosureOfDisaggregationOfRevenueFromContractsWithCustomersLineItems" xlink:label="ifrs-full_DisclosureOfDisaggregationOfRevenueFromContractsWithCustomersLineItems" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" order="1" xlink:from="ifrs-full_DisclosureOfDisaggregationOfRevenueFromContractsWithCustomersLineItems" xlink:to="ifrs-full_DisclosureOfDisaggregationOfRevenueFromContractsWithCustomersTable" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_RevenueFromContractsWithCustomers" xlink:label="ifrs-full_RevenueFromContractsWithCustomers" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="ifrs-full_DisclosureOfDisaggregationOfRevenueFromContractsWithCustomersLineItems" xlink:to="ifrs-full_RevenueFromContractsWithCustomers" priority="1" use="optional" />
  </definitionLink>
  <definitionLink xlink:role="http://acimume.com/role/DisclosureContractRevenuesAndOtherOperatingIncomeAntiAbetaActiveImmunotherapyInAd2024AgreementTakedaPharmaceuticalsUsaIncDetails" xlink:type="extended" xlink:title="40302 - Disclosure - Contract revenues and other operating income - Anti-Abeta Active Immunotherapy in AD - 2024 agreement Takeda Pharmaceuticals, USA Inc. (Details)" xmlns="http://www.xbrl.org/2003/linkbase">
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_DisclosureOfDisaggregationOfRevenueFromContractsWithCustomersTable" xlink:label="ifrs-full_DisclosureOfDisaggregationOfRevenueFromContractsWithCustomersTable" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_TypesOfContractsAxis" xlink:label="ifrs-full_TypesOfContractsAxis" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="1" xlink:from="ifrs-full_DisclosureOfDisaggregationOfRevenueFromContractsWithCustomersTable" xlink:to="ifrs-full_TypesOfContractsAxis" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_TypesOfContractsMember" xlink:label="ifrs-full_TypesOfContractsMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="ifrs-full_TypesOfContractsAxis" xlink:to="ifrs-full_TypesOfContractsMember" use="optional" />
    <loc xlink:type="locator" xlink:href="aciu-20240630.xsd#aciu_AntiAbetaActiveImmunotherapyInAd2024AgreementTakedaPharmaceuticalsUsaInc.Member" xlink:label="aciu_AntiAbetaActiveImmunotherapyInAd2024AgreementTakedaPharmaceuticalsUsaInc.Member" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="ifrs-full_TypesOfContractsMember" xlink:to="aciu_AntiAbetaActiveImmunotherapyInAd2024AgreementTakedaPharmaceuticalsUsaInc.Member" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_RangeAxis" xlink:label="ifrs-full_RangeAxis" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="2" xlink:from="ifrs-full_DisclosureOfDisaggregationOfRevenueFromContractsWithCustomersTable" xlink:to="ifrs-full_RangeAxis" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_RangesMember" xlink:label="ifrs-full_RangesMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="ifrs-full_RangeAxis" xlink:to="ifrs-full_RangesMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_TopOfRangeMember" xlink:label="ifrs-full_TopOfRangeMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="ifrs-full_RangesMember" xlink:to="ifrs-full_TopOfRangeMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_PerformanceObligationsAxis" xlink:label="ifrs-full_PerformanceObligationsAxis" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="3" xlink:from="ifrs-full_DisclosureOfDisaggregationOfRevenueFromContractsWithCustomersTable" xlink:to="ifrs-full_PerformanceObligationsAxis" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_PerformanceObligationsMember" xlink:label="ifrs-full_PerformanceObligationsMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="ifrs-full_PerformanceObligationsAxis" xlink:to="ifrs-full_PerformanceObligationsMember" use="optional" />
    <loc xlink:type="locator" xlink:href="aciu-20240630.xsd#aciu_LicenseOptionAndDevelopmentCmcAndRegulatoryActivitiesMember" xlink:label="aciu_LicenseOptionAndDevelopmentCmcAndRegulatoryActivitiesMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="ifrs-full_PerformanceObligationsMember" xlink:to="aciu_LicenseOptionAndDevelopmentCmcAndRegulatoryActivitiesMember" use="optional" />
    <loc xlink:type="locator" xlink:href="aciu-20240630.xsd#aciu_LicenseOptionMember" xlink:label="aciu_LicenseOptionMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="aciu_LicenseOptionAndDevelopmentCmcAndRegulatoryActivitiesMember" xlink:to="aciu_LicenseOptionMember" use="optional" />
    <loc xlink:type="locator" xlink:href="aciu-20240630.xsd#aciu_DevelopmentCmcAndRegulatoryActivitiesMember" xlink:label="aciu_DevelopmentCmcAndRegulatoryActivitiesMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="aciu_LicenseOptionAndDevelopmentCmcAndRegulatoryActivitiesMember" xlink:to="aciu_DevelopmentCmcAndRegulatoryActivitiesMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_DisclosureOfDisaggregationOfRevenueFromContractsWithCustomersLineItems" xlink:label="ifrs-full_DisclosureOfDisaggregationOfRevenueFromContractsWithCustomersLineItems" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" order="1" xlink:from="ifrs-full_DisclosureOfDisaggregationOfRevenueFromContractsWithCustomersLineItems" xlink:to="ifrs-full_DisclosureOfDisaggregationOfRevenueFromContractsWithCustomersTable" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <loc xlink:type="locator" xlink:href="aciu-20240630.xsd#aciu_CollaborativeAgreementUpfrontPaymentsReceived" xlink:label="aciu_CollaborativeAgreementUpfrontPaymentsReceived" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="ifrs-full_DisclosureOfDisaggregationOfRevenueFromContractsWithCustomersLineItems" xlink:to="aciu_CollaborativeAgreementUpfrontPaymentsReceived" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="aciu-20240630.xsd#aciu_CollaborativeAgreementPotentialReceivablesForOptionExerciseFeeAndPotentialDevelopmentCommercialAndSalesBasedMilestones" xlink:label="aciu_CollaborativeAgreementPotentialReceivablesForOptionExerciseFeeAndPotentialDevelopmentCommercialAndSalesBasedMilestones" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="ifrs-full_DisclosureOfDisaggregationOfRevenueFromContractsWithCustomersLineItems" xlink:to="aciu_CollaborativeAgreementPotentialReceivablesForOptionExerciseFeeAndPotentialDevelopmentCommercialAndSalesBasedMilestones" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="aciu-20240630.xsd#aciu_CollaborativeAgreementNoticeOfContractTerminationPeriod" xlink:label="aciu_CollaborativeAgreementNoticeOfContractTerminationPeriod" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="ifrs-full_DisclosureOfDisaggregationOfRevenueFromContractsWithCustomersLineItems" xlink:to="aciu_CollaborativeAgreementNoticeOfContractTerminationPeriod" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="aciu-20240630.xsd#aciu_NumberOfPerformanceObligationsIdentified" xlink:label="aciu_NumberOfPerformanceObligationsIdentified" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="5" xlink:from="ifrs-full_DisclosureOfDisaggregationOfRevenueFromContractsWithCustomersLineItems" xlink:to="aciu_NumberOfPerformanceObligationsIdentified" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_TransactionPriceAllocatedToRemainingPerformanceObligations" xlink:label="ifrs-full_TransactionPriceAllocatedToRemainingPerformanceObligations" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="6" xlink:from="ifrs-full_DisclosureOfDisaggregationOfRevenueFromContractsWithCustomersLineItems" xlink:to="ifrs-full_TransactionPriceAllocatedToRemainingPerformanceObligations" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="aciu-20240630.xsd#aciu_NumberOfDevelopmentMilestonesThatMayOccurPriorToOptionExerciseIncludedInTransactionPrice" xlink:label="aciu_NumberOfDevelopmentMilestonesThatMayOccurPriorToOptionExerciseIncludedInTransactionPrice" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="7" xlink:from="ifrs-full_DisclosureOfDisaggregationOfRevenueFromContractsWithCustomersLineItems" xlink:to="aciu_NumberOfDevelopmentMilestonesThatMayOccurPriorToOptionExerciseIncludedInTransactionPrice" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_RevenueFromContractsWithCustomers" xlink:label="ifrs-full_RevenueFromContractsWithCustomers" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="8" xlink:from="ifrs-full_DisclosureOfDisaggregationOfRevenueFromContractsWithCustomersLineItems" xlink:to="ifrs-full_RevenueFromContractsWithCustomers" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_ContractLiabilities" xlink:label="ifrs-full_ContractLiabilities" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="9" xlink:from="ifrs-full_DisclosureOfDisaggregationOfRevenueFromContractsWithCustomersLineItems" xlink:to="ifrs-full_ContractLiabilities" priority="1" use="optional" />
  </definitionLink>
  <definitionLink xlink:role="http://acimume.com/role/DisclosureContractRevenuesAndOtherOperatingIncomeGrantsFromMichaelJFoxFoundationDetails" xlink:type="extended" xlink:title="40303 - Disclosure - Contract revenues and other operating income - Grants from the Michael J. Fox Foundation (Details)" xmlns="http://www.xbrl.org/2003/linkbase">
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_DisclosureOfDisaggregationOfRevenueFromContractsWithCustomersTable" xlink:label="ifrs-full_DisclosureOfDisaggregationOfRevenueFromContractsWithCustomersTable" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_TypesOfContractsAxis" xlink:label="ifrs-full_TypesOfContractsAxis" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="1" xlink:from="ifrs-full_DisclosureOfDisaggregationOfRevenueFromContractsWithCustomersTable" xlink:to="ifrs-full_TypesOfContractsAxis" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_TypesOfContractsMember" xlink:label="ifrs-full_TypesOfContractsMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="ifrs-full_TypesOfContractsAxis" xlink:to="ifrs-full_TypesOfContractsMember" use="optional" />
    <loc xlink:type="locator" xlink:href="aciu-20240630.xsd#aciu_GrantRevenueMember" xlink:label="aciu_GrantRevenueMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="ifrs-full_TypesOfContractsMember" xlink:to="aciu_GrantRevenueMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_DisclosureOfDisaggregationOfRevenueFromContractsWithCustomersLineItems" xlink:label="ifrs-full_DisclosureOfDisaggregationOfRevenueFromContractsWithCustomersLineItems" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" order="1" xlink:from="ifrs-full_DisclosureOfDisaggregationOfRevenueFromContractsWithCustomersLineItems" xlink:to="ifrs-full_DisclosureOfDisaggregationOfRevenueFromContractsWithCustomersTable" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_DisclosureOfDisaggregationOfRevenueFromContractsWithCustomersAbstract" xlink:label="ifrs-full_DisclosureOfDisaggregationOfRevenueFromContractsWithCustomersAbstract" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="ifrs-full_DisclosureOfDisaggregationOfRevenueFromContractsWithCustomersLineItems" xlink:to="ifrs-full_DisclosureOfDisaggregationOfRevenueFromContractsWithCustomersAbstract" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_RevenueFromContractsWithCustomers" xlink:label="ifrs-full_RevenueFromContractsWithCustomers" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="ifrs-full_DisclosureOfDisaggregationOfRevenueFromContractsWithCustomersAbstract" xlink:to="ifrs-full_RevenueFromContractsWithCustomers" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_OtherOperatingIncomeExpense" xlink:label="ifrs-full_OtherOperatingIncomeExpense" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="ifrs-full_DisclosureOfDisaggregationOfRevenueFromContractsWithCustomersAbstract" xlink:to="ifrs-full_OtherOperatingIncomeExpense" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_CurrentAccruedIncomeIncludingCurrentContractAssets" xlink:label="ifrs-full_CurrentAccruedIncomeIncludingCurrentContractAssets" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="ifrs-full_DisclosureOfDisaggregationOfRevenueFromContractsWithCustomersAbstract" xlink:to="ifrs-full_CurrentAccruedIncomeIncludingCurrentContractAssets" priority="1" use="optional" />
  </definitionLink>
  <definitionLink xlink:role="http://acimume.com/role/DisclosureLossPerShareAntidilutiveSecuritiesDetails" xlink:type="extended" xlink:title="40402 - Disclosure - Loss per share - Antidilutive securities (Details)" xmlns="http://www.xbrl.org/2003/linkbase">
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_DisclosureOfInstrumentsWithPotentialFutureDilutiveEffectNotIncludedInCalculationOfDilutedEarningsPerShareTable" xlink:label="ifrs-full_DisclosureOfInstrumentsWithPotentialFutureDilutiveEffectNotIncludedInCalculationOfDilutedEarningsPerShareTable" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_TypesOfAntidilutiveInstrumentsAxis" xlink:label="ifrs-full_TypesOfAntidilutiveInstrumentsAxis" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="1" xlink:from="ifrs-full_DisclosureOfInstrumentsWithPotentialFutureDilutiveEffectNotIncludedInCalculationOfDilutedEarningsPerShareTable" xlink:to="ifrs-full_TypesOfAntidilutiveInstrumentsAxis" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_AntidilutiveInstrumentsMember" xlink:label="ifrs-full_AntidilutiveInstrumentsMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="ifrs-full_TypesOfAntidilutiveInstrumentsAxis" xlink:to="ifrs-full_AntidilutiveInstrumentsMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_ShareOptionsMember" xlink:label="ifrs-full_ShareOptionsMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="ifrs-full_AntidilutiveInstrumentsMember" xlink:to="ifrs-full_ShareOptionsMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_RestrictedShareUnitsMember" xlink:label="ifrs-full_RestrictedShareUnitsMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="ifrs-full_AntidilutiveInstrumentsMember" xlink:to="ifrs-full_RestrictedShareUnitsMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_DisclosureOfInstrumentsWithPotentialFutureDilutiveEffectNotIncludedInCalculationOfDilutedEarningsPerShareLineItems" xlink:label="ifrs-full_DisclosureOfInstrumentsWithPotentialFutureDilutiveEffectNotIncludedInCalculationOfDilutedEarningsPerShareLineItems" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" order="1" xlink:from="ifrs-full_DisclosureOfInstrumentsWithPotentialFutureDilutiveEffectNotIncludedInCalculationOfDilutedEarningsPerShareLineItems" xlink:to="ifrs-full_DisclosureOfInstrumentsWithPotentialFutureDilutiveEffectNotIncludedInCalculationOfDilutedEarningsPerShareTable" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_NumberOfInstrumentsThatAreAntidilutiveInPeriodPresented" xlink:label="ifrs-full_NumberOfInstrumentsThatAreAntidilutiveInPeriodPresented" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="ifrs-full_DisclosureOfInstrumentsWithPotentialFutureDilutiveEffectNotIncludedInCalculationOfDilutedEarningsPerShareLineItems" xlink:to="ifrs-full_NumberOfInstrumentsThatAreAntidilutiveInPeriodPresented" priority="1" use="optional" />
  </definitionLink>
  <definitionLink xlink:role="http://acimume.com/role/DisclosurePropertyPlantAndEquipmentDetails" xlink:type="extended" xlink:title="40501 - Disclosure - Property, plant and equipment (Details)" xmlns="http://www.xbrl.org/2003/linkbase">
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_DisclosureOfPropertyPlantAndEquipmentTable" xlink:label="ifrs-full_DisclosureOfPropertyPlantAndEquipmentTable" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_ClassesOfPropertyPlantAndEquipmentAxis" xlink:label="ifrs-full_ClassesOfPropertyPlantAndEquipmentAxis" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="1" xlink:from="ifrs-full_DisclosureOfPropertyPlantAndEquipmentTable" xlink:to="ifrs-full_ClassesOfPropertyPlantAndEquipmentAxis" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_PropertyPlantAndEquipmentMember" xlink:label="ifrs-full_PropertyPlantAndEquipmentMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="ifrs-full_ClassesOfPropertyPlantAndEquipmentAxis" xlink:to="ifrs-full_PropertyPlantAndEquipmentMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_FixturesAndFittingsMember" xlink:label="ifrs-full_FixturesAndFittingsMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="ifrs-full_PropertyPlantAndEquipmentMember" xlink:to="ifrs-full_FixturesAndFittingsMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_ComputerEquipmentMember" xlink:label="ifrs-full_ComputerEquipmentMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="ifrs-full_PropertyPlantAndEquipmentMember" xlink:to="ifrs-full_ComputerEquipmentMember" use="optional" />
    <loc xlink:type="locator" xlink:href="aciu-20240630.xsd#aciu_LaboratoryEquipmentMember" xlink:label="aciu_LaboratoryEquipmentMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="ifrs-full_PropertyPlantAndEquipmentMember" xlink:to="aciu_LaboratoryEquipmentMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_LeaseholdImprovementsMember" xlink:label="ifrs-full_LeaseholdImprovementsMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="ifrs-full_PropertyPlantAndEquipmentMember" xlink:to="ifrs-full_LeaseholdImprovementsMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_CarryingAmountAccumulatedDepreciationAmortisationAndImpairmentAndGrossCarryingAmountAxis" xlink:label="ifrs-full_CarryingAmountAccumulatedDepreciationAmortisationAndImpairmentAndGrossCarryingAmountAxis" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="2" xlink:from="ifrs-full_DisclosureOfPropertyPlantAndEquipmentTable" xlink:to="ifrs-full_CarryingAmountAccumulatedDepreciationAmortisationAndImpairmentAndGrossCarryingAmountAxis" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_CarryingAmountMember" xlink:label="ifrs-full_CarryingAmountMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="ifrs-full_CarryingAmountAccumulatedDepreciationAmortisationAndImpairmentAndGrossCarryingAmountAxis" xlink:to="ifrs-full_CarryingAmountMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_GrossCarryingAmountMember" xlink:label="ifrs-full_GrossCarryingAmountMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="ifrs-full_CarryingAmountMember" xlink:to="ifrs-full_GrossCarryingAmountMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_AccumulatedDepreciationAndAmortisationMember" xlink:label="ifrs-full_AccumulatedDepreciationAndAmortisationMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="ifrs-full_CarryingAmountMember" xlink:to="ifrs-full_AccumulatedDepreciationAndAmortisationMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_DisclosureOfPropertyPlantAndEquipmentLineItems" xlink:label="ifrs-full_DisclosureOfPropertyPlantAndEquipmentLineItems" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" order="1" xlink:from="ifrs-full_DisclosureOfPropertyPlantAndEquipmentLineItems" xlink:to="ifrs-full_DisclosureOfPropertyPlantAndEquipmentTable" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_ReconciliationOfChangesInPropertyPlantAndEquipmentAbstract" xlink:label="ifrs-full_ReconciliationOfChangesInPropertyPlantAndEquipmentAbstract" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="ifrs-full_DisclosureOfPropertyPlantAndEquipmentLineItems" xlink:to="ifrs-full_ReconciliationOfChangesInPropertyPlantAndEquipmentAbstract" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_PropertyPlantAndEquipment" xlink:label="ifrs-full_PropertyPlantAndEquipment" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="ifrs-full_ReconciliationOfChangesInPropertyPlantAndEquipmentAbstract" xlink:to="ifrs-full_PropertyPlantAndEquipment" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_AdditionsOtherThanThroughBusinessCombinationsPropertyPlantAndEquipment" xlink:label="ifrs-full_AdditionsOtherThanThroughBusinessCombinationsPropertyPlantAndEquipment" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="ifrs-full_ReconciliationOfChangesInPropertyPlantAndEquipmentAbstract" xlink:to="ifrs-full_AdditionsOtherThanThroughBusinessCombinationsPropertyPlantAndEquipment" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_DepreciationPropertyPlantAndEquipment" xlink:label="ifrs-full_DepreciationPropertyPlantAndEquipment" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="ifrs-full_ReconciliationOfChangesInPropertyPlantAndEquipmentAbstract" xlink:to="ifrs-full_DepreciationPropertyPlantAndEquipment" priority="1" use="optional" />
  </definitionLink>
  <definitionLink xlink:role="http://acimume.com/role/DisclosureRightOfUseAssetsLongTermFinancialAssetsAndLeaseLiabilitiesDetails" xlink:type="extended" xlink:title="40601 - Disclosure - Right-of-use assets, long-term financial assets and lease liabilities (Details)" xmlns="http://www.xbrl.org/2003/linkbase">
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_DisclosureOfQuantitativeInformationAboutRightofuseAssetsTable" xlink:label="ifrs-full_DisclosureOfQuantitativeInformationAboutRightofuseAssetsTable" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_ClassesOfAssetsAxis" xlink:label="ifrs-full_ClassesOfAssetsAxis" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="1" xlink:from="ifrs-full_DisclosureOfQuantitativeInformationAboutRightofuseAssetsTable" xlink:to="ifrs-full_ClassesOfAssetsAxis" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_ClassesOfAssetsMember" xlink:label="ifrs-full_ClassesOfAssetsMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="ifrs-full_ClassesOfAssetsAxis" xlink:to="ifrs-full_ClassesOfAssetsMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_PropertyPlantAndEquipmentMember" xlink:label="ifrs-full_PropertyPlantAndEquipmentMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="ifrs-full_ClassesOfAssetsMember" xlink:to="ifrs-full_PropertyPlantAndEquipmentMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_BuildingsMember" xlink:label="ifrs-full_BuildingsMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="ifrs-full_PropertyPlantAndEquipmentMember" xlink:to="ifrs-full_BuildingsMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_OfficeEquipmentMember" xlink:label="ifrs-full_OfficeEquipmentMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="ifrs-full_PropertyPlantAndEquipmentMember" xlink:to="ifrs-full_OfficeEquipmentMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_ComputerEquipmentMember" xlink:label="ifrs-full_ComputerEquipmentMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="ifrs-full_PropertyPlantAndEquipmentMember" xlink:to="ifrs-full_ComputerEquipmentMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_RangeAxis" xlink:label="ifrs-full_RangeAxis" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="2" xlink:from="ifrs-full_DisclosureOfQuantitativeInformationAboutRightofuseAssetsTable" xlink:to="ifrs-full_RangeAxis" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_RangesMember" xlink:label="ifrs-full_RangesMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="ifrs-full_RangeAxis" xlink:to="ifrs-full_RangesMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_TopOfRangeMember" xlink:label="ifrs-full_TopOfRangeMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="ifrs-full_RangesMember" xlink:to="ifrs-full_TopOfRangeMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_DisclosureOfQuantitativeInformationAboutRightofuseAssetsLineItems" xlink:label="ifrs-full_DisclosureOfQuantitativeInformationAboutRightofuseAssetsLineItems" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" order="1" xlink:from="ifrs-full_DisclosureOfQuantitativeInformationAboutRightofuseAssetsLineItems" xlink:to="ifrs-full_DisclosureOfQuantitativeInformationAboutRightofuseAssetsTable" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_DisclosureOfQuantitativeInformationAboutLeasesForLesseeAbstract" xlink:label="ifrs-full_DisclosureOfQuantitativeInformationAboutLeasesForLesseeAbstract" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="ifrs-full_DisclosureOfQuantitativeInformationAboutRightofuseAssetsLineItems" xlink:to="ifrs-full_DisclosureOfQuantitativeInformationAboutLeasesForLesseeAbstract" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_WeightedAverageLesseesIncrementalBorrowingRateAppliedToLeaseLiabilitiesRecognisedAtDateOfInitialApplicationOfIFRS16" xlink:label="ifrs-full_WeightedAverageLesseesIncrementalBorrowingRateAppliedToLeaseLiabilitiesRecognisedAtDateOfInitialApplicationOfIFRS16" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="ifrs-full_DisclosureOfQuantitativeInformationAboutLeasesForLesseeAbstract" xlink:to="ifrs-full_WeightedAverageLesseesIncrementalBorrowingRateAppliedToLeaseLiabilitiesRecognisedAtDateOfInitialApplicationOfIFRS16" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="aciu-20240630.xsd#aciu_RightOfUseLeasedAssetsAbstract" xlink:label="aciu_RightOfUseLeasedAssetsAbstract" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="ifrs-full_DisclosureOfQuantitativeInformationAboutLeasesForLesseeAbstract" xlink:to="aciu_RightOfUseLeasedAssetsAbstract" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_RightofuseAssets" xlink:label="ifrs-full_RightofuseAssets" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="aciu_RightOfUseLeasedAssetsAbstract" xlink:to="ifrs-full_RightofuseAssets" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_AdditionsToRightofuseAssets" xlink:label="ifrs-full_AdditionsToRightofuseAssets" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="aciu_RightOfUseLeasedAssetsAbstract" xlink:to="ifrs-full_AdditionsToRightofuseAssets" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_DepreciationRightofuseAssets" xlink:label="ifrs-full_DepreciationRightofuseAssets" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="aciu_RightOfUseLeasedAssetsAbstract" xlink:to="ifrs-full_DepreciationRightofuseAssets" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_ExpenseRelatingToVariableLeasePaymentsNotIncludedInMeasurementOfLeaseLiabilities" xlink:label="ifrs-full_ExpenseRelatingToVariableLeasePaymentsNotIncludedInMeasurementOfLeaseLiabilities" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="ifrs-full_DisclosureOfQuantitativeInformationAboutLeasesForLesseeAbstract" xlink:to="ifrs-full_ExpenseRelatingToVariableLeasePaymentsNotIncludedInMeasurementOfLeaseLiabilities" priority="1" use="optional" />
  </definitionLink>
  <definitionLink xlink:role="http://acimume.com/role/DisclosureRightOfUseAssetsLongTermFinancialAssetsAndLeaseLiabilitiesContractualUndiscountedCashFlowsForLeaseObligationsDetails" xlink:type="extended" xlink:title="40603 - Disclosure - Right-of-use assets, long-term financial assets and lease liabilities - Contractual undiscounted cash flows for lease obligations (Details)" xmlns="http://www.xbrl.org/2003/linkbase">
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_DisclosureOfMaturityAnalysisOfOperatingLeasePaymentsTable" xlink:label="ifrs-full_DisclosureOfMaturityAnalysisOfOperatingLeasePaymentsTable" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_MaturityAxis" xlink:label="ifrs-full_MaturityAxis" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="1" xlink:from="ifrs-full_DisclosureOfMaturityAnalysisOfOperatingLeasePaymentsTable" xlink:to="ifrs-full_MaturityAxis" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_AggregatedTimeBandsMember" xlink:label="ifrs-full_AggregatedTimeBandsMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="ifrs-full_MaturityAxis" xlink:to="ifrs-full_AggregatedTimeBandsMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_NotLaterThanOneYearMember" xlink:label="ifrs-full_NotLaterThanOneYearMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="ifrs-full_AggregatedTimeBandsMember" xlink:to="ifrs-full_NotLaterThanOneYearMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_LaterThanOneYearAndNotLaterThanThreeYearsMember" xlink:label="ifrs-full_LaterThanOneYearAndNotLaterThanThreeYearsMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="ifrs-full_AggregatedTimeBandsMember" xlink:to="ifrs-full_LaterThanOneYearAndNotLaterThanThreeYearsMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_LaterThanThreeYearsAndNotLaterThanFiveYearsMember" xlink:label="ifrs-full_LaterThanThreeYearsAndNotLaterThanFiveYearsMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="ifrs-full_AggregatedTimeBandsMember" xlink:to="ifrs-full_LaterThanThreeYearsAndNotLaterThanFiveYearsMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_DisclosureOfMaturityAnalysisOfOperatingLeasePaymentsLineItems" xlink:label="ifrs-full_DisclosureOfMaturityAnalysisOfOperatingLeasePaymentsLineItems" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" order="1" xlink:from="ifrs-full_DisclosureOfMaturityAnalysisOfOperatingLeasePaymentsLineItems" xlink:to="ifrs-full_DisclosureOfMaturityAnalysisOfOperatingLeasePaymentsTable" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <loc xlink:type="locator" xlink:href="aciu-20240630.xsd#aciu_ContractualUndiscountedCashFlowsForLeaseLiabilitiesAbstract" xlink:label="aciu_ContractualUndiscountedCashFlowsForLeaseLiabilitiesAbstract" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="ifrs-full_DisclosureOfMaturityAnalysisOfOperatingLeasePaymentsLineItems" xlink:to="aciu_ContractualUndiscountedCashFlowsForLeaseLiabilitiesAbstract" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_GrossLeaseLiabilities" xlink:label="ifrs-full_GrossLeaseLiabilities" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="aciu_ContractualUndiscountedCashFlowsForLeaseLiabilitiesAbstract" xlink:to="ifrs-full_GrossLeaseLiabilities" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="aciu-20240630.xsd#aciu_EscrowDepositAmount" xlink:label="aciu_EscrowDepositAmount" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="aciu_ContractualUndiscountedCashFlowsForLeaseLiabilitiesAbstract" xlink:to="aciu_EscrowDepositAmount" priority="1" use="optional" />
  </definitionLink>
  <definitionLink xlink:role="http://acimume.com/role/DisclosureIntangibleAssetsDetails" xlink:type="extended" xlink:title="40801 - Disclosure - Intangible assets (Details)" xmlns="http://www.xbrl.org/2003/linkbase">
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_DisclosureOfIntangibleAssetsTable" xlink:label="ifrs-full_DisclosureOfIntangibleAssetsTable" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_ClassesOfIntangibleAssetsOtherThanGoodwillAxis" xlink:label="ifrs-full_ClassesOfIntangibleAssetsOtherThanGoodwillAxis" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="1" xlink:from="ifrs-full_DisclosureOfIntangibleAssetsTable" xlink:to="ifrs-full_ClassesOfIntangibleAssetsOtherThanGoodwillAxis" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_IntangibleAssetsOtherThanGoodwillMember" xlink:label="ifrs-full_IntangibleAssetsOtherThanGoodwillMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="ifrs-full_ClassesOfIntangibleAssetsOtherThanGoodwillAxis" xlink:to="ifrs-full_IntangibleAssetsOtherThanGoodwillMember" use="optional" />
    <loc xlink:type="locator" xlink:href="aciu-20240630.xsd#aciu_InProcessResearchAndDevelopmentIPRDAssetMember" xlink:label="aciu_InProcessResearchAndDevelopmentIPRDAssetMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="ifrs-full_IntangibleAssetsOtherThanGoodwillMember" xlink:to="aciu_InProcessResearchAndDevelopmentIPRDAssetMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_CarryingAmountAccumulatedDepreciationAmortisationAndImpairmentAndGrossCarryingAmountAxis" xlink:label="ifrs-full_CarryingAmountAccumulatedDepreciationAmortisationAndImpairmentAndGrossCarryingAmountAxis" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="2" xlink:from="ifrs-full_DisclosureOfIntangibleAssetsTable" xlink:to="ifrs-full_CarryingAmountAccumulatedDepreciationAmortisationAndImpairmentAndGrossCarryingAmountAxis" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_CarryingAmountMember" xlink:label="ifrs-full_CarryingAmountMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="ifrs-full_CarryingAmountAccumulatedDepreciationAmortisationAndImpairmentAndGrossCarryingAmountAxis" xlink:to="ifrs-full_CarryingAmountMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_GrossCarryingAmountMember" xlink:label="ifrs-full_GrossCarryingAmountMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="ifrs-full_CarryingAmountMember" xlink:to="ifrs-full_GrossCarryingAmountMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_DisclosureOfIntangibleAssetsLineItems" xlink:label="ifrs-full_DisclosureOfIntangibleAssetsLineItems" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" order="1" xlink:from="ifrs-full_DisclosureOfIntangibleAssetsLineItems" xlink:to="ifrs-full_DisclosureOfIntangibleAssetsTable" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_DisclosureOfIntangibleAssetsMaterialToEntityAbstract" xlink:label="ifrs-full_DisclosureOfIntangibleAssetsMaterialToEntityAbstract" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="ifrs-full_DisclosureOfIntangibleAssetsLineItems" xlink:to="ifrs-full_DisclosureOfIntangibleAssetsMaterialToEntityAbstract" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_IntangibleAssetsOtherThanGoodwill" xlink:label="ifrs-full_IntangibleAssetsOtherThanGoodwill" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="ifrs-full_DisclosureOfIntangibleAssetsMaterialToEntityAbstract" xlink:to="ifrs-full_IntangibleAssetsOtherThanGoodwill" priority="1" use="optional" />
  </definitionLink>
  <definitionLink xlink:role="http://acimume.com/role/DisclosureShareCapitalAndTreasurySharesDetails" xlink:type="extended" xlink:title="41201 - Disclosure - Share capital and Treasury shares (Details)" xmlns="http://www.xbrl.org/2003/linkbase">
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_DisclosureOfClassesOfShareCapitalTable" xlink:label="ifrs-full_DisclosureOfClassesOfShareCapitalTable" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_CounterpartiesAxis" xlink:label="ifrs-full_CounterpartiesAxis" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="1" xlink:from="ifrs-full_DisclosureOfClassesOfShareCapitalTable" xlink:to="ifrs-full_CounterpartiesAxis" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_CounterpartiesMember" xlink:label="ifrs-full_CounterpartiesMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="ifrs-full_CounterpartiesAxis" xlink:to="ifrs-full_CounterpartiesMember" use="optional" />
    <loc xlink:type="locator" xlink:href="aciu-20240630.xsd#aciu_AcImmuneUsaMember" xlink:label="aciu_AcImmuneUsaMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="ifrs-full_CounterpartiesMember" xlink:to="aciu_AcImmuneUsaMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_DisclosureOfClassesOfShareCapitalLineItems" xlink:label="ifrs-full_DisclosureOfClassesOfShareCapitalLineItems" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" order="1" xlink:from="ifrs-full_DisclosureOfClassesOfShareCapitalLineItems" xlink:to="ifrs-full_DisclosureOfClassesOfShareCapitalTable" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <loc xlink:type="locator" xlink:href="aciu-20240630.xsd#aciu_NumberOfSharesIssuedDuringPeriod" xlink:label="aciu_NumberOfSharesIssuedDuringPeriod" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="ifrs-full_DisclosureOfClassesOfShareCapitalLineItems" xlink:to="aciu_NumberOfSharesIssuedDuringPeriod" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="aciu-20240630.xsd#aciu_NumberOfTreasurySharesSoldDuringPeriod" xlink:label="aciu_NumberOfTreasurySharesSoldDuringPeriod" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="ifrs-full_DisclosureOfClassesOfShareCapitalLineItems" xlink:to="aciu_NumberOfTreasurySharesSoldDuringPeriod" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_ProceedsFromSaleOrIssueOfTreasuryShares" xlink:label="ifrs-full_ProceedsFromSaleOrIssueOfTreasuryShares" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="ifrs-full_DisclosureOfClassesOfShareCapitalLineItems" xlink:to="ifrs-full_ProceedsFromSaleOrIssueOfTreasuryShares" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2023-03-23/full_ifrs/full_ifrs-cor_2023-03-23.xsd#ifrs-full_SharesInEntityHeldByEntityOrByItsSubsidiariesOrAssociates" xlink:label="ifrs-full_SharesInEntityHeldByEntityOrByItsSubsidiariesOrAssociates" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="5" xlink:from="ifrs-full_DisclosureOfClassesOfShareCapitalLineItems" xlink:to="ifrs-full_SharesInEntityHeldByEntityOrByItsSubsidiariesOrAssociates" priority="1" use="optional" />
  </definitionLink>
</link:linkbase>
