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<!-- DFIN ActiveDisclosure(SM) XBRL Schema Document - http://www.dfinsolutions.com/ -->
<!-- Creation Date      : 2021-03-25T00:04:05.6417066+00:00 -->
<!-- Version            : 5.0.1.571 -->
<!-- Package ID         : 7ade75fdc1d245c2b73727bcd69694ea -->
<!-- Copyright (c) 2021 Donnelley Financial Solutions, Inc. All Rights Reserved. -->
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  <xsd:element id="nktx_LeaseExpirationMonthAndYear" name="LeaseExpirationMonthAndYear" type="xbrli:gYearMonthItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="nktx_LesseeOperatingLeaseLeaseNotYetCommencedMinimumLeasePayments" name="LesseeOperatingLeaseLeaseNotYetCommencedMinimumLeasePayments" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="nktx_AgreementWithFinancialInstitutionMember" name="AgreementWithFinancialInstitutionMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="nktx_LetterOfCreditNoticePeriodByFinancialInstitutionBeforeAnnualExpirationDate" name="LetterOfCreditNoticePeriodByFinancialInstitutionBeforeAnnualExpirationDate" type="xbrli:durationItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="nktx_LicenseAgreementUpfrontPayment" name="LicenseAgreementUpfrontPayment" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="nktx_StockIssuedDuringPeriodSharesIssuedUpfrontPayments" name="StockIssuedDuringPeriodSharesIssuedUpfrontPayments" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="nktx_AnnualLicenseMaintenanceFee" name="AnnualLicenseMaintenanceFee" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="nktx_PotentialMilestonePayments" name="PotentialMilestonePayments" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="nktx_PercentageOfLicensorsRoyalties" name="PercentageOfLicensorsRoyalties" type="num:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="nktx_PercentageOfSublicensingIncome" name="PercentageOfSublicensingIncome" type="num:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="nktx_LicenseAgreementTerm" name="LicenseAgreementTerm" type="xbrli:durationItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="nktx_FairValueOfCommonStockIssued" name="FairValueOfCommonStockIssued" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="nktx_LicenseAgreementLineItems" name="LicenseAgreementLineItems" type="xbrli:stringItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="nktx_LicenseAgreementTable" name="LicenseAgreementTable" type="xbrli:stringItemType" substitutionGroup="xbrldt:hypercubeItem" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="nktx_NationalUniversityOfSingaporeAndStJudeChildrenSResearchHospitalMember" name="NationalUniversityOfSingaporeAndStJudeChildrenSResearchHospitalMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="nktx_LicenseAgreementMember" name="LicenseAgreementMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="nktx_AfterYearTwoMember" name="AfterYearTwoMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="nktx_MilestonePaymentUponCompletionOfRegulatoryMilestoneRelatedToClinicalDevelopment" name="MilestonePaymentUponCompletionOfRegulatoryMilestoneRelatedToClinicalDevelopment" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="nktx_KPlanMember" name="KPlanMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="nktx_SeriesAConvertiblePreferredStockMember" name="SeriesAConvertiblePreferredStockMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="nktx_SeriesAFinancingInvestorsMember" name="SeriesAFinancingInvestorsMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="nktx_GlaxoSmithKlineConvertibleNoteMember" name="GlaxoSmithKlineConvertibleNoteMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="nktx_SeriesAInvestorsMember" name="SeriesAInvestorsMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="nktx_SeriesBConvertiblePromissoryNotesMember" name="SeriesBConvertiblePromissoryNotesMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="nktx_StockPurchaseAgreementMember" name="StockPurchaseAgreementMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="nktx_PercentageOfOutstandingConvertibleNotesConvertedIntoConvertiblePreferredStock" name="PercentageOfOutstandingConvertibleNotesConvertedIntoConvertiblePreferredStock" type="num:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="nktx_ConvertibleNotesAccruedInterestPerYear" name="ConvertibleNotesAccruedInterestPerYear" type="num:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="nktx_PercentageOfConvertibleNotesAutomaticallyConvertIntoSeriesBConvertiblePreferredStock" name="PercentageOfConvertibleNotesAutomaticallyConvertIntoSeriesBConvertiblePreferredStock" type="num:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="nktx_AmortizationTermOfInterestExpenseUsingEffectiveInterestMethod" name="AmortizationTermOfInterestExpenseUsingEffectiveInterestMethod" type="xbrli:durationItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="nktx_ChangeInConversionBenefitResultedInAdjustmentToDerivativeInstrument" name="ChangeInConversionBenefitResultedInAdjustmentToDerivativeInstrument" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="nktx_TemporaryEquityStockIssuableUponAchievementOfMilestone" name="TemporaryEquityStockIssuableUponAchievementOfMilestone" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="nktx_GrossProceedsFromIssuanceOfConvertiblePreferredStock" name="GrossProceedsFromIssuanceOfConvertiblePreferredStock" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="nktx_ConversionOfStockSharesConvertible" name="ConversionOfStockSharesConvertible" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" />
  <xsd:element id="nktx_CommonStockNumberOfVotingRightsForEachShare" name="CommonStockNumberOfVotingRightsForEachShare" type="xbrli:integerItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" />
  <xsd:element id="nktx_TwoThousandFifteenEquityIncentivePlanMember" name="TwoThousandFifteenEquityIncentivePlanMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="nktx_TwoThousandTwentyPerformanceIncentivePlanMember" name="TwoThousandTwentyPerformanceIncentivePlanMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="nktx_SeparationAndReleaseAgreementMember" name="SeparationAndReleaseAgreementMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="nktx_IncreaseInShareLimitAsPercentageOfOutstandingSharesOfCommonStockOnLastTradingDayInPriorYear" name="IncreaseInShareLimitAsPercentageOfOutstandingSharesOfCommonStockOnLastTradingDayInPriorYear" type="num:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="nktx_TwoThousandTwentyPerformanceIncentiveAndTwoThousandFifteenEquityIncentivePlanMember" name="TwoThousandTwentyPerformanceIncentiveAndTwoThousandFifteenEquityIncentivePlanMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="nktx_ShareBasedCompensationArrangementByShareBasedPaymentAwardOfferingPeriod" name="ShareBasedCompensationArrangementByShareBasedPaymentAwardOfferingPeriod" type="xbrli:durationItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="nktx_AgreementExecutionDate" name="AgreementExecutionDate" type="xbrli:dateItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="nktx_PeriodWithinSeveranceBenefitsPayableAfterTerminationDate" name="PeriodWithinSeveranceBenefitsPayableAfterTerminationDate" type="xbrli:durationItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="nktx_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsCommonStockFairValue" name="ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsCommonStockFairValue" type="num:perShareItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="nktx_IncomeTaxesLineItems" name="IncomeTaxesLineItems" type="xbrli:stringItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="nktx_IncomeTaxesTable" name="IncomeTaxesTable" type="xbrli:stringItemType" substitutionGroup="xbrldt:hypercubeItem" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="nktx_ExpirationYearOneMember" name="ExpirationYearOneMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="nktx_ExpirationYearTwoMember" name="ExpirationYearTwoMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="nktx_CarryForwardIndefinitelyMember" name="CarryForwardIndefinitelyMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="nktx_NetOperatingLossAndTaxCreditCarryforwardsToOffsetMember" name="NetOperatingLossAndTaxCreditCarryforwardsToOffsetMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="nktx_IncomeTaxReconciliationPermanentItems" name="IncomeTaxReconciliationPermanentItems" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="nktx_IncomeTaxReconciliationChangeInFairValueOfDerivativeLiability" name="IncomeTaxReconciliationChangeInFairValueOfDerivativeLiability" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="nktx_DeferredTaxAssetsDepreciationAndAmortization" name="DeferredTaxAssetsDepreciationAndAmortization" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="nktx_DeferredTaxAssetsLeaseLiability" name="DeferredTaxAssetsLeaseLiability" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="nktx_DeferredTaxLiabilitiesRightOfUseAsset" name="DeferredTaxLiabilitiesRightOfUseAsset" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="nktx_DeferredTaxLiabilitiesDepreciationAndAmortization" name="DeferredTaxLiabilitiesDepreciationAndAmortization" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="nktx_OperatingLossCarryforwardsExpirationYear" name="OperatingLossCarryforwardsExpirationYear" type="xbrli:gYearItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="nktx_TaxCreditCarryforwardExpirationYear" name="TaxCreditCarryforwardExpirationYear" type="xbrli:gYearItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="nktx_ChangesInOwnershipByStockholdersHoldingPercentage" name="ChangesInOwnershipByStockholdersHoldingPercentage" type="num:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
</xsd:schema>
