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<!-- DFIN ActiveDisclosure(SM) XBRL Schema Document - http://www.dfinsolutions.com/ -->
<!-- Creation Date      : 2020-03-27T03:36:00.1020115+00:00 -->
<!-- Version            : 5.0.1.369 -->
<!-- Package ID         : a8bd74ff5af3449e94d1e8468da3b8a3 -->
<!-- Copyright (c) 2020 Donnelley Financial Solutions, Inc. All Rights Reserved. -->
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  <xsd:element id="alt_StockIssuedDuringPeriodValueExerciseOfWarrants" name="StockIssuedDuringPeriodValueExerciseOfWarrants" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="alt_SeriesBRedeemableConvertiblePreferredStockMember" name="SeriesBRedeemableConvertiblePreferredStockMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="alt_AdjustmentsToAdditionalPaidInCapitalShareBasedCompensationRequisiteServicePeriodRecognitionShares" name="AdjustmentsToAdditionalPaidInCapitalShareBasedCompensationRequisiteServicePeriodRecognitionShares" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="alt_TemporaryEquityStockIssuedDuringPeriodSharesNewIssues" name="TemporaryEquityStockIssuedDuringPeriodSharesNewIssues" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="alt_DebtDiscountAccretion" name="DebtDiscountAccretion" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="alt_IncreaseDecreaseInLeaseObligations" name="IncreaseDecreaseInLeaseObligations" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="alt_ProceedsFromConditionalEconomicIncentive" name="ProceedsFromConditionalEconomicIncentive" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="alt_ProceedsFromExercisesOfWarrantsAndStockOptions" name="ProceedsFromExercisesOfWarrantsAndStockOptions" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="alt_CommonStockIssuedForAcquiredInProcessResearchAndDevelopment" name="CommonStockIssuedForAcquiredInProcessResearchAndDevelopment" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="alt_SettlementOfWarrantLiabilityForCommonStock" name="SettlementOfWarrantLiabilityForCommonStock" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="alt_NotesPayableRelatedToExchangeOfWarrants" name="NotesPayableRelatedToExchangeOfWarrants" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="alt_NotesPayableAndOtherLiabilities" name="NotesPayableAndOtherLiabilities" type="nonnum:textBlockItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="alt_NotesPayableAndOtherLiabilitiesDisclosureAbstract" name="NotesPayableAndOtherLiabilitiesDisclosureAbstract" type="xbrli:stringItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="alt_CommonStockTextBlock" name="CommonStockTextBlock" type="nonnum:textBlockItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="alt_WarrantsPolicyTextBlock" name="WarrantsPolicyTextBlock" type="nonnum:textBlockItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="alt_ContingentConsiderationPolicyTextBlock" name="ContingentConsiderationPolicyTextBlock" type="nonnum:textBlockItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="alt_PropertyAndEquipmentEstimatedUsefulLivesTableTextBlock" name="PropertyAndEquipmentEstimatedUsefulLivesTableTextBlock" type="nonnum:textBlockItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="alt_ScheduleOfIntangibleAssetsTableTextBlock" name="ScheduleOfIntangibleAssetsTableTextBlock" type="nonnum:textBlockItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="alt_ScheduleOfWarrantsOutstandingAndExercisableTableTextBlock" name="ScheduleOfWarrantsOutstandingAndExercisableTableTextBlock" type="nonnum:textBlockItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="alt_MonteCarloSimulationModelMember" name="MonteCarloSimulationModelMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="alt_KeyAssumptionsUsedInCalculatingMarketabilityDiscountTableTextBlock" name="KeyAssumptionsUsedInCalculatingMarketabilityDiscountTableTextBlock" type="nonnum:textBlockItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="alt_ScheduleOfIncomeStatementEffectOfChangesInOutstandingWarrantsTableTextBlock" name="ScheduleOfIncomeStatementEffectOfChangesInOutstandingWarrantsTableTextBlock" type="nonnum:textBlockItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="alt_ScheduleOfSupplementalBalanceSheetInformationRelatedToOperatingLeasesTableTextBlock" name="ScheduleOfSupplementalBalanceSheetInformationRelatedToOperatingLeasesTableTextBlock" type="nonnum:textBlockItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="alt_SignificantAccountingPoliciesLineItems" name="SignificantAccountingPoliciesLineItems" type="xbrli:stringItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="alt_SignificantAccountingPoliciesTable" name="SignificantAccountingPoliciesTable" type="xbrli:stringItemType" substitutionGroup="xbrldt:hypercubeItem" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="alt_ReleaseInRestrictedCash" name="ReleaseInRestrictedCash" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="alt_LaboratoryEquipmentMember" name="LaboratoryEquipmentMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="alt_NewAccountingPronouncementOrChangeInAccountingPrincipleCumulativeEffectOfChangeOnAssets" name="NewAccountingPronouncementOrChangeInAccountingPrincipleCumulativeEffectOfChangeOnAssets" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="alt_NewAccountingPronouncementOrChangeInAccountingPrincipleCumulativeEffectOfChangeOnLiabilities" name="NewAccountingPronouncementOrChangeInAccountingPrincipleCumulativeEffectOfChangeOnLiabilities" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="alt_SpitfirePharmaIncMember" name="SpitfirePharmaIncMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="alt_InvestigationalNewDrugApplicationMilestoneMember" name="InvestigationalNewDrugApplicationMilestoneMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="alt_InvestigationalNewDrugApplicationAndRegulatoryMilestonesMember" name="InvestigationalNewDrugApplicationAndRegulatoryMilestonesMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="alt_SalesMilestoneMember" name="SalesMilestoneMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="alt_RegulatoryMilestoneMember" name="RegulatoryMilestoneMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="alt_NumberOfConsecutiveTradingDays" name="NumberOfConsecutiveTradingDays" type="xbrli:durationItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="alt_PercentageOfUpfrontConsiderationReleaseAtPeriodOne" name="PercentageOfUpfrontConsiderationReleaseAtPeriodOne" type="num:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="alt_PercentageOfUpfrontConsiderationReleaseAtPeriodTwo" name="PercentageOfUpfrontConsiderationReleaseAtPeriodTwo" type="num:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="alt_PercentageOfUpfrontConsiderationReleaseAtPeriodThree" name="PercentageOfUpfrontConsiderationReleaseAtPeriodThree" type="num:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="alt_BusinessCombinationContingentConsiderationPaymentTerm" name="BusinessCombinationContingentConsiderationPaymentTerm" type="xbrli:durationItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="alt_BusinessAcquisitionTermOfLicenseAgreement" name="BusinessAcquisitionTermOfLicenseAgreement" type="xbrli:durationItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="alt_PercentageOfUpfrontConsiderationReleaseAtPeriodFour" name="PercentageOfUpfrontConsiderationReleaseAtPeriodFour" type="num:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="alt_OtherDilutiveSecuritiesMember" name="OtherDilutiveSecuritiesMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="alt_ScheduleOfIntangibleAssetsLineItems" name="ScheduleOfIntangibleAssetsLineItems" type="xbrli:stringItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="alt_ScheduleOfIntangibleAssetsTable" name="ScheduleOfIntangibleAssetsTable" type="xbrli:stringItemType" substitutionGroup="xbrldt:hypercubeItem" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="alt_InternallyDevelopedPatentsMember" name="InternallyDevelopedPatentsMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="alt_AcquiredLicensesMember" name="AcquiredLicensesMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="alt_OncosynCancerImmunotherapyMember" name="OncosynCancerImmunotherapyMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="alt_HepTcellAndSparVaxLMember" name="HepTcellAndSparVaxLMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="alt_HepTcellMember" name="HepTcellMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="alt_SparVaxLMember" name="SparVaxLMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="alt_NumberOfAssets" name="NumberOfAssets" type="xbrli:integerItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" />
  <xsd:element id="alt_AccruedResearchAndDevelopment" name="AccruedResearchAndDevelopment" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="alt_UniversityOfAlabamaAtBirminghamResearchFoundationMember" name="UniversityOfAlabamaAtBirminghamResearchFoundationMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="alt_JanssenVaccinesAndPreventionBvMember" name="JanssenVaccinesAndPreventionBvMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="alt_AuburnUniversityMember" name="AuburnUniversityMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="alt_LicenseAgreementExpirationPeriod" name="LicenseAgreementExpirationPeriod" type="xbrli:durationItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="alt_LicenseAgreementExpirationDate" name="LicenseAgreementExpirationDate" type="xbrli:dateItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="alt_UpfrontFees" name="UpfrontFees" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="alt_BPIFranceNotesMember" name="BPIFranceNotesMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="alt_EconomicConditionalIncentiveGrants" name="EconomicConditionalIncentiveGrants" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="alt_NumberOfNonInterestBearingResearchAndDevelopmentFundingArrangements" name="NumberOfNonInterestBearingResearchAndDevelopmentFundingArrangements" type="xbrli:integerItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="alt_PercentageOfFundToBeWithdrawnUponSigningOfArrangements" name="PercentageOfFundToBeWithdrawnUponSigningOfArrangements" type="num:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="alt_AdditionalPercentageOfFundToBeWithdrawnContingentUponFinancialAuditAndTechnicalProgressReport" name="AdditionalPercentageOfFundToBeWithdrawnContingentUponFinancialAuditAndTechnicalProgressReport" type="num:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="alt_ResearchAndDevelopmentFundingArrangementsExtendedPeriod" name="ResearchAndDevelopmentFundingArrangementsExtendedPeriod" type="xbrli:durationItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="alt_PaidInExcessOfAllowableFundingAmount" name="PaidInExcessOfAllowableFundingAmount" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="alt_ResearchFundingFirstArrangementMember" name="ResearchFundingFirstArrangementMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="alt_ResearchFundingSecondArrangementMember" name="ResearchFundingSecondArrangementMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="alt_MontgomeryCountyMember" name="MontgomeryCountyMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="alt_MontgomeryCountyAndStateOfMarylandMember" name="MontgomeryCountyAndStateOfMarylandMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="alt_FirstWarrantsExchangeMember" name="FirstWarrantsExchangeMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="alt_NumberOfEconomicConditionalIncentiveGrants" name="NumberOfEconomicConditionalIncentiveGrants" type="xbrli:integerItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="alt_EconomicConditionalIncentiveGrantExpirationDate" name="EconomicConditionalIncentiveGrantExpirationDate" type="xbrli:dateItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="alt_EconomicConditionalIncentiveGrantTerm" name="EconomicConditionalIncentiveGrantTerm" type="xbrli:durationItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="alt_AccruedInterestRateOnGrant" name="AccruedInterestRateOnGrant" type="num:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="alt_WeightedAveragePricesOfCommonStock" name="WeightedAveragePricesOfCommonStock" type="num:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="alt_CommonStockWeightedAveragePricePerShare" name="CommonStockWeightedAveragePricePerShare" type="num:perShareItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="alt_AdditionalInterestEarned" name="AdditionalInterestEarned" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="alt_SecondWarrantsExchangeMember" name="SecondWarrantsExchangeMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="alt_DeemedDividend" name="DeemedDividend" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="alt_DeemedCapitalContribution" name="DeemedCapitalContribution" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="alt_PaymentForOutstandingPreferredShares" name="PaymentForOutstandingPreferredShares" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="alt_ChangesInEquityAndComprehensiveIncomeLineItemsLineItems" name="ChangesInEquityAndComprehensiveIncomeLineItemsLineItems" type="xbrli:stringItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="alt_FirstRegisteredDirectOfferingMember" name="FirstRegisteredDirectOfferingMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="alt_UnitOfferingMember" name="UnitOfferingMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="alt_CommonUnitsMember" name="CommonUnitsMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="alt_PrefundedUnitsMember" name="PrefundedUnitsMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="alt_SecondRegisteredDirectOfferingMember" name="SecondRegisteredDirectOfferingMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="alt_ThirdRegisteredDirectOfferingMember" name="ThirdRegisteredDirectOfferingMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="alt_ClassOfWarrantOrRightExpirationPeriod" name="ClassOfWarrantOrRightExpirationPeriod" type="xbrli:durationItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="alt_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights2" name="ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights2" type="num:perShareItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" />
  <xsd:element id="alt_ClassOfWarrantOrRightsIssuanceDate" name="ClassOfWarrantOrRightsIssuanceDate" type="xbrli:dateItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="alt_ClassesOrWarrantRightExpirationDate" name="ClassesOrWarrantRightExpirationDate" type="xbrli:dateItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="alt_ReplacementWarrantsMember" name="ReplacementWarrantsMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="alt_WarrantsIssuedWithRedeemablePreferredMember" name="WarrantsIssuedWithRedeemablePreferredMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="alt_WarrantsIssuedWithCommonUnitsInUnitOfferingMember" name="WarrantsIssuedWithCommonUnitsInUnitOfferingMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="alt_UnderwriterWarrantIssuedWithPublicOfferingMember" name="UnderwriterWarrantIssuedWithPublicOfferingMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="alt_IssuedWithCommonUnitsInSecondRegisteredDirectOfferingMember" name="IssuedWithCommonUnitsInSecondRegisteredDirectOfferingMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="alt_IssuedWithCommonUnitsInRegisteredDirectOfferingMember" name="IssuedWithCommonUnitsInRegisteredDirectOfferingMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="alt_IssuedWithCommonUnitsInThirdRegisteredDirectOfferingMember" name="IssuedWithCommonUnitsInThirdRegisteredDirectOfferingMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="alt_ReplacementWarrantsIssuedInConnectionWithMergers" name="ReplacementWarrantsIssuedInConnectionWithMergers" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="alt_ThirdWarrantsExchangeMember" name="ThirdWarrantsExchangeMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="alt_ConversionOfWarrantsIntoCommonStockAndCash" name="ConversionOfWarrantsIntoCommonStockAndCash" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="alt_GainLossOnExchangeOfWarrants" name="GainLossOnExchangeOfWarrants" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="alt_WarrantsTransactionCost" name="WarrantsTransactionCost" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="alt_CommonStockAdditionalSharesIssued" name="CommonStockAdditionalSharesIssued" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" />
  <xsd:element id="alt_LossOnExchangeOfWarrants" name="LossOnExchangeOfWarrants" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="alt_TransactionCostsAllocatedToWarrants" name="TransactionCostsAllocatedToWarrants" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="alt_ChangeInFairValueOfWarrantLiabilityIncludingGainLossOnExchange" name="ChangeInFairValueOfWarrantLiabilityIncludingGainLossOnExchange" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="alt_NumberOfWarrantsExercisedExchangedRepurchasedForConversionOfSecurities" name="NumberOfWarrantsExercisedExchangedRepurchasedForConversionOfSecurities" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="alt_DerivativeLiabilitiesRecognizedAsResultOfSettlementAgreement" name="DerivativeLiabilitiesRecognizedAsResultOfSettlementAgreement" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="alt_TwoThousandAndOneEmployeeStockOptionPlanMember" name="TwoThousandAndOneEmployeeStockOptionPlanMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="alt_StockIncentivePlanTwentyZeroSevenMember" name="StockIncentivePlanTwentyZeroSevenMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="alt_TwoThousandSeventeenOmnibusIncentivePlanMember" name="TwoThousandSeventeenOmnibusIncentivePlanMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="alt_TwoThousandAndEighteenInducementGrantPlanMember" name="TwoThousandAndEighteenInducementGrantPlanMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="alt_OctoberTwoThousandSixteenMember" name="OctoberTwoThousandSixteenMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="alt_NovemberThirtyTwoThousandEighteenMember" name="NovemberThirtyTwoThousandEighteenMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="alt_TwoThousandAndNineteenEmployeeStockPurchasePlanMember" name="TwoThousandAndNineteenEmployeeStockPurchasePlanMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="alt_PercentageOfAdditionalSharesFromCommonStockAvailableForStockBasedCompensation" name="PercentageOfAdditionalSharesFromCommonStockAvailableForStockBasedCompensation" type="num:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" />
  <xsd:element id="alt_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedAndExpectedToVestOutstandingNumber" name="ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedAndExpectedToVestOutstandingNumber" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" />
  <xsd:element id="alt_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesVestingDate" name="ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesVestingDate" type="xbrli:dateItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="alt_EffectiveIncomeTaxRateReconciliationSharebasedCompensationCostPercent" name="EffectiveIncomeTaxRateReconciliationSharebasedCompensationCostPercent" type="num:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="alt_EffectiveIncomeTaxRateReconciliationAcquisitionCosts" name="EffectiveIncomeTaxRateReconciliationAcquisitionCosts" type="num:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="alt_EffectiveIncomeTaxRateReconciliationNondeductibleExpenseGoodwillImpairmentLossesRate" name="EffectiveIncomeTaxRateReconciliationNondeductibleExpenseGoodwillImpairmentLossesRate" type="num:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="alt_EffectiveIncomeTaxRateReconciliationNondeductibleExpenseAcquiredInProcessResearchAndDevelopmentImpairment" name="EffectiveIncomeTaxRateReconciliationNondeductibleExpenseAcquiredInProcessResearchAndDevelopmentImpairment" type="num:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="alt_EffectiveIncomeTaxRateReconciliationLossOnWarrantExchangeAndWarrantFairMarketValueChanges" name="EffectiveIncomeTaxRateReconciliationLossOnWarrantExchangeAndWarrantFairMarketValueChanges" type="num:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="alt_DeferredTaxAssetsAmortization" name="DeferredTaxAssetsAmortization" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="alt_DeferredTaxAssetsLeaseLiability" name="DeferredTaxAssetsLeaseLiability" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="alt_DeferredTaxLiabilitiesRightOfUseAsset" name="DeferredTaxLiabilitiesRightOfUseAsset" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="alt_DeferredTaxLiabilitiesDepreciation" name="DeferredTaxLiabilitiesDepreciation" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="alt_DeferredTaxLiabilitiesAccruedCompensationIrcSection481a" name="DeferredTaxLiabilitiesAccruedCompensationIrcSection481a" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="alt_DeferredTaxAssetsNetBeforeValuationAllowance" name="DeferredTaxAssetsNetBeforeValuationAllowance" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="alt_OperatingLossCarryforwardsNotSubjectToExpiration" name="OperatingLossCarryforwardsNotSubjectToExpiration" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="alt_OperatingLossCarryforwardsSubjectToExpiration" name="OperatingLossCarryforwardsSubjectToExpiration" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="alt_OperatingLossCarryForwardsExpirationPeriod" name="OperatingLossCarryForwardsExpirationPeriod" type="xbrli:durationItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="alt_IncomeTaxesLineItems" name="IncomeTaxesLineItems" type="xbrli:stringItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="alt_IncomeTaxesTable" name="IncomeTaxesTable" type="xbrli:stringItemType" substitutionGroup="xbrldt:hypercubeItem" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="alt_IncreaseInOwnershipPercentage" name="IncreaseInOwnershipPercentage" type="num:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="alt_TestingPeriod" name="TestingPeriod" type="xbrli:durationItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="alt_NonCancellableLeaseMaturityPeriod" name="NonCancellableLeaseMaturityPeriod" type="xbrli:gYearMonthItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="alt_AmortizationOfLeaseIncentivesBenefit" name="AmortizationOfLeaseIncentivesBenefit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="alt_CommitmentsAndContingenciesLineItems" name="CommitmentsAndContingenciesLineItems" type="xbrli:stringItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="alt_CommitmentsAndContingenciesTable" name="CommitmentsAndContingenciesTable" type="xbrli:stringItemType" substitutionGroup="xbrldt:hypercubeItem" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="alt_MeasurementInputCostOfCapitalMember" name="MeasurementInputCostOfCapitalMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
</xsd:schema>
