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  <xsd:element name="CommercialPaperAndCorporateDebtSecuritiesMember" id="alt_CommercialPaperAndCorporateDebtSecuritiesMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration" />
  <xsd:element name="AssetsOrLiabilitiesMeasuredAtFairValueOnNonRecurringBasis" id="alt_AssetsOrLiabilitiesMeasuredAtFairValueOnNonRecurringBasis" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" abstract="false" nillable="true" xbrli:balance="credit" xbrli:periodType="instant" />
  <xsd:element name="AccruedResearchAndDevelopment" id="alt_AccruedResearchAndDevelopment" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" abstract="false" nillable="true" xbrli:balance="credit" xbrli:periodType="instant" />
  <xsd:element name="SpitfirePharmaIncMember" id="alt_SpitfirePharmaIncMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration" />
  <xsd:element name="SalesMilestoneMember" id="alt_SalesMilestoneMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration" />
  <xsd:element name="BusinessAcquisitionTermOfLicenseAgreement" id="alt_BusinessAcquisitionTermOfLicenseAgreement" type="xbrli:durationItemType" substitutionGroup="xbrli:item" abstract="false" nillable="true" xbrli:periodType="duration" />
  <xsd:element name="EconomicConditionalIncentiveGrants" id="alt_EconomicConditionalIncentiveGrants" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" abstract="false" nillable="true" xbrli:balance="credit" xbrli:periodType="instant" />
  <xsd:element name="ChangesInEquityAndComprehensiveIncomeLineItemsLineItems" id="alt_ChangesInEquityAndComprehensiveIncomeLineItemsLineItems" type="xbrli:stringItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration" />
  <xsd:element name="PreFundedWarrantsMember" id="alt_PreFundedWarrantsMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration" />
  <xsd:element name="AtTheMarketOfferingMember" id="alt_AtTheMarketOfferingMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration" />
  <xsd:element name="AggregateOfferingPrice" id="alt_AggregateOfferingPrice" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" abstract="false" nillable="true" xbrli:balance="credit" xbrli:periodType="duration" />
  <xsd:element name="TwoThousandAndNineteenEmployeeStockPurchasePlanMember" id="alt_TwoThousandAndNineteenEmployeeStockPurchasePlanMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration" />
  <xsd:element name="CommitmentsAndContingenciesLineItems" id="alt_CommitmentsAndContingenciesLineItems" type="xbrli:stringItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration" />
  <xsd:element name="CommitmentsAndContingenciesTable" id="alt_CommitmentsAndContingenciesTable" type="xbrli:stringItemType" substitutionGroup="xbrldt:hypercubeItem" abstract="true" nillable="true" xbrli:periodType="duration" />
  <xsd:element name="RegisteredDirectOfferingMember" id="alt_RegisteredDirectOfferingMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration" />
  <xsd:element name="TwoThousandSeventeenOmnibusIncentivePlanMember" id="alt_TwoThousandSeventeenOmnibusIncentivePlanMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration" />
  <xsd:element name="PercentageOfAdditionalSharesFromCommonStockAvailableForStockBasedCompensation" id="alt_PercentageOfAdditionalSharesFromCommonStockAvailableForStockBasedCompensation" type="dtr-types:percentItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="instant" />
  <xsd:element name="IncomeTaxesAndResearchDevelopmentIncentiveReceivableCurrent" id="alt_IncomeTaxesAndResearchDevelopmentIncentiveReceivableCurrent" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="instant" abstract="false" xbrli:balance="debit" />
  <xsd:element name="IncreaseDecreaseInIncomeTaxesAndTaxIncentiveReceivable" id="alt_IncreaseDecreaseInIncomeTaxesAndTaxIncentiveReceivable" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="false" xbrli:balance="credit" />
  <xsd:element name="ResearchAndDevelopmentIncentiveCredit" id="alt_ResearchAndDevelopmentIncentiveCredit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="instant" abstract="false" xbrli:balance="credit" />
  <xsd:element name="DeferredOfferingCostsInAccruedExpensesAndOtherCurrentLiabilities" id="alt_DeferredOfferingCostsInAccruedExpensesAndOtherCurrentLiabilities" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="false" xbrli:balance="debit" />
  <xsd:element name="EvercoreGroupAndJmpSecuritiesLlcAndBRileySecuritiesMember" id="alt_EvercoreGroupAndJmpSecuritiesLlcAndBRileySecuritiesMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="true" />
  <xsd:element name="CommonStockVotingRightPerShare" id="alt_CommonStockVotingRightPerShare" type="xbrli:integerItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="instant" abstract="false" />
  <xsd:element name="Rule10b51ArrModifiedFlag" id="alt_Rule10b51ArrModifiedFlag" type="xbrli:booleanItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="false" />
  <xsd:element name="NonRule10b51ArrModifiedFlag" id="alt_NonRule10b51ArrModifiedFlag" type="xbrli:booleanItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="false" />
  <xsd:element name="PublicOfferingRemainingAmountAvailableForSale" id="alt_PublicOfferingRemainingAmountAvailableForSale" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="instant" abstract="false" xbrli:balance="credit" />
  <xsd:element name="LeerinkPartnersLlcPiperSandlerCo.AndStifelNicolausCompanyIncorporatedMember" id="alt_LeerinkPartnersLlcPiperSandlerCo.AndStifelNicolausCompanyIncorporatedMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="true" />
  <xsd:element name="DebtInstrumentInterestPaymentTerm" id="alt_DebtInstrumentInterestPaymentTerm" type="xbrli:durationItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="false" />
  <xsd:element name="OtherNoncurrentLiabilitiesDisclosureTextBlock" id="alt_OtherNoncurrentLiabilitiesDisclosureTextBlock" type="dtr-types:textBlockItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="false" />
  <xsd:element name="DebtInstrumentSecuredDebtTrancheOneMember" id="alt_DebtInstrumentSecuredDebtTrancheOneMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="true" />
  <xsd:element name="DebtInstrumentSecuredDebtTrancheTwoMember" id="alt_DebtInstrumentSecuredDebtTrancheTwoMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="true" />
  <xsd:element name="DebtInstrumentSecuredDebtTrancheThreeMember" id="alt_DebtInstrumentSecuredDebtTrancheThreeMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="true" />
  <xsd:element name="DebtInstrumentSecuredDebtTrancheFourMember" id="alt_DebtInstrumentSecuredDebtTrancheFourMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="true" />
  <xsd:element name="InterestRateUntilDecember312025Member" id="alt_InterestRateUntilDecember312025Member" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="true" />
  <xsd:element name="PrepaymentsWithinTwelveMonthsAfterClosingMember" id="alt_PrepaymentsWithinTwelveMonthsAfterClosingMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="true" />
  <xsd:element name="PrepaymentsWithinTwentyFourMonthsAfterClosingMember" id="alt_PrepaymentsWithinTwentyFourMonthsAfterClosingMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="true" />
  <xsd:element name="PrepaymentsAfterThirtySixMonthsOfClosingMember" id="alt_PrepaymentsAfterThirtySixMonthsOfClosingMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="true" />
  <xsd:element name="DebtInstrumentMaximumNumberOfTranches" id="alt_DebtInstrumentMaximumNumberOfTranches" type="xbrli:integerItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="instant" abstract="false" />
  <xsd:element name="DebtInstrumentPrepaymentChargeOnPrincipalBalancePercentage" id="alt_DebtInstrumentPrepaymentChargeOnPrincipalBalancePercentage" type="dtr-types:percentItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="instant" abstract="false" />
  <xsd:element name="DebtInstrumentPrepaymentTerm" id="alt_DebtInstrumentPrepaymentTerm" type="xbrli:durationItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="false" />
  <xsd:element name="DebtInstrumentEndOfTermChargePercentage" id="alt_DebtInstrumentEndOfTermChargePercentage" type="dtr-types:percentItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="instant" abstract="false" />
  <xsd:element name="DebtInstrumentFacilityChargePercentage" id="alt_DebtInstrumentFacilityChargePercentage" type="dtr-types:percentItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="false" />
  <xsd:element name="LongTermDebtEndOfTermCharge" id="alt_LongTermDebtEndOfTermCharge" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="instant" abstract="false" xbrli:balance="credit" />
  <xsd:element name="PrepaymentsWithinThirtySixMonthsAfterClosingMember" id="alt_PrepaymentsWithinThirtySixMonthsAfterClosingMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="true" />
  <xsd:element name="DebtInstrumentCovenantMinimumMarketCapitalization" id="alt_DebtInstrumentCovenantMinimumMarketCapitalization" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="instant" abstract="false" xbrli:balance="credit" />
  <xsd:element name="LeerinkPartnersLlcMember" id="alt_LeerinkPartnersLlcMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="true" />
  <xsd:element name="WarrantsIssuedDuringPeriod" id="alt_WarrantsIssuedDuringPeriod" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="false" />
  <xsd:element name="ProceedsFromIssuanceOfCommonStockAndPreFundedWarrantsDirectOfferingNet" id="alt_ProceedsFromIssuanceOfCommonStockAndPreFundedWarrantsDirectOfferingNet" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="false" xbrli:balance="debit" />
  <xsd:element name="April2026DirectOfferingMember" id="alt_April2026DirectOfferingMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="true" />
  <xsd:element name="ClassOfWarrantOrRightCombinedOfferingPriceOfWarrantsOrRights" id="alt_ClassOfWarrantOrRightCombinedOfferingPriceOfWarrantsOrRights" type="dtr-types:perShareItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="instant" abstract="false" />
  <xsd:element name="AdditionalProceedsFromIssuanceOrSaleOfEquity" id="alt_AdditionalProceedsFromIssuanceOrSaleOfEquity" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="false" xbrli:balance="debit" />
  <xsd:element name="ScenarioAllCommonStockWarrantsSoldExercisedInCashMember" id="alt_ScenarioAllCommonStockWarrantsSoldExercisedInCashMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="true" />
  <xsd:element name="CommonStockWarrantMember" id="alt_CommonStockWarrantMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="true" />
  <xsd:element name="CombinedOfferingPriceOfEachShareOfCommonStockAndCommonStockWarrant" id="alt_CombinedOfferingPriceOfEachShareOfCommonStockAndCommonStockWarrant" type="dtr-types:perShareItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="instant" abstract="false" />
  <xsd:element name="CommonStockWarrantsOneMember" id="alt_CommonStockWarrantsOneMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="true" />
  <xsd:element name="CommonStockWarrantsTwoMember" id="alt_CommonStockWarrantsTwoMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="true" />
</xsd:schema>
