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  <xsd:element name="CommitmentsAndContingenciesTable" id="alt_CommitmentsAndContingenciesTable" type="xbrli:stringItemType" substitutionGroup="xbrldt:hypercubeItem" abstract="true" nillable="true" xbrli:periodType="duration" />
  <xsd:element name="LonzaManufacturingAgreementMember" id="alt_LonzaManufacturingAgreementMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration" />
  <xsd:element name="DevelopmentBasedMilestonePayments" id="alt_DevelopmentBasedMilestonePayments" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" abstract="false" nillable="true" xbrli:balance="credit" xbrli:periodType="duration" />
  <xsd:element name="CommitmentToProcureEquipmentAndConstruction" id="alt_CommitmentToProcureEquipmentAndConstruction" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" abstract="false" nillable="true" xbrli:balance="credit" 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="CommonStockIssuedRelatedToContingentConsiderationLiability" id="alt_CommonStockIssuedRelatedToContingentConsiderationLiability" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:balance="credit" xbrli:periodType="duration" />
  <xsd:element name="AdditionOperatingLeaseLiabilityAndRightOfUseAsset" id="alt_AdditionOperatingLeaseLiabilityAndRightOfUseAsset" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:balance="debit" xbrli:periodType="duration" />
  <xsd:element name="NotesPayableAndOtherLiabilitiesDisclosureAbstract" id="alt_NotesPayableAndOtherLiabilitiesDisclosureAbstract" type="xbrli:stringItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration" />
  <xsd:element name="NotesPayableAndOtherLiabilities" id="alt_NotesPayableAndOtherLiabilities" type="dtr-types:textBlockItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" />
  <xsd:element name="WarrantsPolicyTextBlock" id="alt_WarrantsPolicyTextBlock" type="dtr-types:textBlockItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" />
  <xsd:element name="ContingentConsiderationPolicyTextBlock" id="alt_ContingentConsiderationPolicyTextBlock" type="dtr-types:textBlockItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" />
  <xsd:element name="PropertyAndEquipmentEstimatedUsefulLivesTableTextBlock" id="alt_PropertyAndEquipmentEstimatedUsefulLivesTableTextBlock" type="dtr-types:textBlockItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" />
  <xsd:element name="ScheduleOfWarrantsOutstandingAndExercisableTableTextBlock" id="alt_ScheduleOfWarrantsOutstandingAndExercisableTableTextBlock" type="dtr-types:textBlockItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" />
  <xsd:element name="SignificantAccountingPoliciesTable" id="alt_SignificantAccountingPoliciesTable" type="xbrli:stringItemType" substitutionGroup="xbrldt:hypercubeItem" abstract="true" nillable="true" xbrli:periodType="duration" />
  <xsd:element name="SignificantAccountingPoliciesLineItems" id="alt_SignificantAccountingPoliciesLineItems" type="xbrli:stringItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration" />
  <xsd:element name="ClinicalTrialNotificationMember" id="alt_ClinicalTrialNotificationMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration" />
  <xsd:element name="SharesIssuedForContingentMilestonePayment" id="alt_SharesIssuedForContingentMilestonePayment" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" />
  <xsd:element name="MontgomeryCountyMember" id="alt_MontgomeryCountyMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration" />
  <xsd:element name="MontgomeryCountyAndStateOfMarylandMember" id="alt_MontgomeryCountyAndStateOfMarylandMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration" />
  <xsd:element name="NumberOfEconomicConditionalIncentiveGrants" id="alt_NumberOfEconomicConditionalIncentiveGrants" type="xbrli:integerItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" />
  <xsd:element name="EconomicConditionalIncentiveGrantExpirationDate" id="alt_EconomicConditionalIncentiveGrantExpirationDate" type="xbrli:dateItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" />
  <xsd:element name="EconomicConditionalIncentiveGrantTerm" id="alt_EconomicConditionalIncentiveGrantTerm" type="xbrli:durationItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" />
  <xsd:element name="AccruedInterestRateOnGrant" id="alt_AccruedInterestRateOnGrant" type="dtr-types:percentItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" />
  <xsd:element name="CommonUnitsMember" id="alt_CommonUnitsMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration" />
  <xsd:element name="ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights2" id="alt_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights2" type="dtr-types:perShareItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="instant" />
  <xsd:element name="ClassOfWarrantOrRightExpirationPeriod" id="alt_ClassOfWarrantOrRightExpirationPeriod" type="xbrli:durationItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" />
  <xsd:element name="ReplacementWarrantsMember" id="alt_ReplacementWarrantsMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration" />
  <xsd:element name="WarrantsIssuedWithCommonUnitsInTwoThousandEighteenUnitOfferingMember" id="alt_WarrantsIssuedWithCommonUnitsInTwoThousandEighteenUnitOfferingMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration" />
  <xsd:element name="IssuedWithCommonUnitsInTwoThousandEighteenRegisteredDirectOfferingMember" id="alt_IssuedWithCommonUnitsInTwoThousandEighteenRegisteredDirectOfferingMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration" />
  <xsd:element name="ClassOfWarrantOrRightsIssuanceDate" id="alt_ClassOfWarrantOrRightsIssuanceDate" type="xbrli:dateItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" />
  <xsd:element name="ClassesOrWarrantRightExpirationDate" id="alt_ClassesOrWarrantRightExpirationDate" type="xbrli:dateItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" />
  <xsd:element name="TwoThousandEighteenUnitOfferingMember" id="alt_TwoThousandEighteenUnitOfferingMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration" />
  <xsd:element name="TwoThousandEighteenRegisteredDirectOfferingMember" id="alt_TwoThousandEighteenRegisteredDirectOfferingMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration" />
  <xsd:element name="TwoThousandAndOneEmployeeStockOptionPlanMember" id="alt_TwoThousandAndOneEmployeeStockOptionPlanMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration" />
  <xsd:element name="StockIncentivePlanTwentyZeroSevenMember" id="alt_StockIncentivePlanTwentyZeroSevenMember" 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="TwoThousandAndEighteenInducementGrantPlanMember" id="alt_TwoThousandAndEighteenInducementGrantPlanMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration" />
  <xsd:element name="NovemberThirtyTwoThousandEighteenMember" id="alt_NovemberThirtyTwoThousandEighteenMember" 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="ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesVestingDate" id="alt_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesVestingDate" type="xbrli:dateItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" />
  <xsd:element name="EffectiveIncomeTaxRateReconciliationNondeductibleExpenseAcquiredInProcessResearchAndDevelopment" id="alt_EffectiveIncomeTaxRateReconciliationNondeductibleExpenseAcquiredInProcessResearchAndDevelopment" type="dtr-types:percentItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" />
  <xsd:element name="DeferredTaxAssetsAmortization" id="alt_DeferredTaxAssetsAmortization" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:balance="debit" xbrli:periodType="instant" />
  <xsd:element name="DeferredTaxAssetsLeaseLiability" id="alt_DeferredTaxAssetsLeaseLiability" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:balance="debit" xbrli:periodType="instant" />
  <xsd:element name="DeferredTaxLiabilitiesRightOfUseAsset" id="alt_DeferredTaxLiabilitiesRightOfUseAsset" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:balance="credit" xbrli:periodType="instant" />
  <xsd:element name="DeferredTaxLiabilitiesDepreciation" id="alt_DeferredTaxLiabilitiesDepreciation" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:balance="credit" xbrli:periodType="instant" />
  <xsd:element name="IncomeTaxesTable" id="alt_IncomeTaxesTable" type="xbrli:stringItemType" substitutionGroup="xbrldt:hypercubeItem" abstract="true" nillable="true" xbrli:periodType="duration" />
  <xsd:element name="DomesticCountryAndStateAndLocalJurisdictionMember" id="alt_DomesticCountryAndStateAndLocalJurisdictionMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration" />
  <xsd:element name="IncomeTaxesLineItems" id="alt_IncomeTaxesLineItems" type="xbrli:stringItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration" />
  <xsd:element name="OperatingLossCarryforwardsSubjectToExpiration" id="alt_OperatingLossCarryforwardsSubjectToExpiration" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:balance="debit" xbrli:periodType="instant" />
  <xsd:element name="OperatingLossCarryForwardsExpirationPeriod" id="alt_OperatingLossCarryForwardsExpirationPeriod" type="xbrli:durationItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" />
  <xsd:element name="OperatingLossCarryforwardsNotSubjectToExpiration" id="alt_OperatingLossCarryforwardsNotSubjectToExpiration" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:balance="debit" xbrli:periodType="instant" />
  <xsd:element name="IncreaseInOwnershipPercentage" id="alt_IncreaseInOwnershipPercentage" type="dtr-types:percentItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" />
  <xsd:element name="TestingPeriod" id="alt_TestingPeriod" type="xbrli:durationItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" />
  <xsd:element name="JanssenVaccinesAndPreventionBvMember" id="alt_JanssenVaccinesAndPreventionBvMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration" />
  <xsd:element name="LicenseAndRoyaltyFeesMember" id="alt_LicenseAndRoyaltyFeesMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration" />
  <xsd:element name="LicenseFeeCosts" id="alt_LicenseFeeCosts" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:balance="credit" xbrli:periodType="duration" />
  <xsd:element name="AmountRequiredToBePayUponReachingCertainMilestone" id="alt_AmountRequiredToBePayUponReachingCertainMilestone" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:balance="debit" xbrli:periodType="duration" />
  <xsd:element name="IssuedInExchangeForRetirementOfCommonStockPerExchangeAgreementMember" id="alt_IssuedInExchangeForRetirementOfCommonStockPerExchangeAgreementMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="true" />
  <xsd:element name="IssuedWithCommonUnitsIn2019RegisteredDirectOfferingMember" id="alt_IssuedWithCommonUnitsIn2019RegisteredDirectOfferingMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="true" />
  <xsd:element name="IssuedWithCommonUnitsIn2020PublicOfferingMember" id="alt_IssuedWithCommonUnitsIn2020PublicOfferingMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="true" />
  <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="ResearchAndDevelopmentIncentiveCreditsPolicyPolicyTextBlock" id="alt_ResearchAndDevelopmentIncentiveCreditsPolicyPolicyTextBlock" type="dtr-types:textBlockItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="false" />
  <xsd:element name="ResearchAndDevelopmentCredits" id="alt_ResearchAndDevelopmentCredits" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="false" xbrli:balance="credit" />
  <xsd:element name="WarrantsExpired" id="alt_WarrantsExpired" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="false" />
  <xsd:element name="PhaseTwoMilestoneMember" id="alt_PhaseTwoMilestoneMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="true" />
  <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="ResearchAndDevelopmentIncentiveCreditEarned" id="alt_ResearchAndDevelopmentIncentiveCreditEarned" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="false" xbrli:balance="credit" />
  <xsd:element name="NumberOfEqualMonthlyInstallments" id="alt_NumberOfEqualMonthlyInstallments" type="xbrli:integerItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="instant" abstract="false" />
  <xsd:element name="IncomeTaxExpenseMember" id="alt_IncomeTaxExpenseMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="true" />
  <xsd:element name="DeferredTaxAssetsCapitalizedResearchAndDevelopmentCosts" id="alt_DeferredTaxAssetsCapitalizedResearchAndDevelopmentCosts" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="instant" abstract="false" xbrli:balance="debit" />
  <xsd:element name="ResearchAndDevelopmentTaxIncentiveReceivable" id="alt_ResearchAndDevelopmentTaxIncentiveReceivable" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="instant" abstract="false" xbrli:balance="debit" />
  <xsd:element name="ExcessTaxRefundPayable" id="alt_ExcessTaxRefundPayable" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="instant" abstract="false" xbrli:balance="credit" />
</xsd:schema>
