<?xml version="1.0" encoding="UTF-8"?>

<!--XBRL Document Created with Wdesk from Workiva-->
<!--Copyright 2022 Workiva-->
<!--r:e419d988-d94a-4293-9bef-79872bf37ae3,g:12854048-adf0-4a0d-ac77-d09eddf7f9be-->
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema" xmlns:link="http://www.xbrl.org/2003/linkbase" xmlns:selb="http://www.selectabio.com/20220630" xmlns:xbrli="http://www.xbrl.org/2003/instance" xmlns:dtr-types="http://www.xbrl.org/dtr/type/2020-01-21" attributeFormDefault="unqualified" elementFormDefault="qualified" targetNamespace="http://www.selectabio.com/20220630">
  <xs:import namespace="http://fasb.org/srt/2022" schemaLocation="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd"/>
  <xs:import namespace="http://fasb.org/us-gaap/2022" schemaLocation="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd"/>
  <xs:import namespace="http://www.w3.org/1999/xlink" schemaLocation="http://www.xbrl.org/2003/xlink-2003-12-31.xsd"/>
  <xs:import namespace="http://www.xbrl.org/2003/instance" schemaLocation="http://www.xbrl.org/2003/xbrl-instance-2003-12-31.xsd"/>
  <xs:import namespace="http://www.xbrl.org/2003/linkbase" schemaLocation="http://www.xbrl.org/2003/xbrl-linkbase-2003-12-31.xsd"/>
  <xs:import namespace="http://www.xbrl.org/dtr/type/2020-01-21" schemaLocation="https://www.xbrl.org/dtr/type/2020-01-21/types.xsd"/>
  <xs:import namespace="http://xbrl.sec.gov/country/2022" schemaLocation="https://xbrl.sec.gov/country/2022/country-2022.xsd"/>
  <xs:import namespace="http://xbrl.sec.gov/dei/2022" schemaLocation="https://xbrl.sec.gov/dei/2022/dei-2022.xsd"/>
  <xs:annotation>
    <xs:appinfo>
      <link:linkbaseRef xmlns:xlink="http://www.w3.org/1999/xlink" xlink:arcrole="http://www.w3.org/1999/xlink/properties/linkbase" xlink:href="selb-20220630_lab.xml" xlink:role="http://www.xbrl.org/2003/role/labelLinkbaseRef" xlink:type="simple"/>
      <link:linkbaseRef xmlns:xlink="http://www.w3.org/1999/xlink" xlink:arcrole="http://www.w3.org/1999/xlink/properties/linkbase" xlink:href="selb-20220630_pre.xml" xlink:role="http://www.xbrl.org/2003/role/presentationLinkbaseRef" xlink:type="simple"/>
      <link:linkbaseRef xmlns:xlink="http://www.w3.org/1999/xlink" xlink:arcrole="http://www.w3.org/1999/xlink/properties/linkbase" xlink:href="selb-20220630_cal.xml" xlink:role="http://www.xbrl.org/2003/role/calculationLinkbaseRef" xlink:type="simple"/>
      <link:linkbaseRef xmlns:xlink="http://www.w3.org/1999/xlink" xlink:arcrole="http://www.w3.org/1999/xlink/properties/linkbase" xlink:href="selb-20220630_def.xml" xlink:role="http://www.xbrl.org/2003/role/definitionLinkbaseRef" xlink:type="simple"/>
      <link:roleType id="CoverPage" roleURI="http://www.selectabio.com/role/CoverPage">
        <link:definition>0001001 - Document - Cover Page</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="ConsolidatedBalanceSheets" roleURI="http://www.selectabio.com/role/ConsolidatedBalanceSheets">
        <link:definition>1001002 - Statement - Consolidated Balance Sheets</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="ConsolidatedBalanceSheetsParenthetical" roleURI="http://www.selectabio.com/role/ConsolidatedBalanceSheetsParenthetical">
        <link:definition>1002003 - Statement - Consolidated Balance Sheets (Parenthetical)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="ConsolidatedStatementsofOperationsandComprehensiveIncomeLoss" roleURI="http://www.selectabio.com/role/ConsolidatedStatementsofOperationsandComprehensiveIncomeLoss">
        <link:definition>1003004 - Statement - Consolidated Statements of Operations and Comprehensive Income (Loss)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="ConsolidatedStatementsofChangesinStockholdersDeficitEquity" roleURI="http://www.selectabio.com/role/ConsolidatedStatementsofChangesinStockholdersDeficitEquity">
        <link:definition>1004005 - Statement - Consolidated Statements of Changes in Stockholders&#8217; (Deficit) Equity</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="ConsolidatedStatementsofCashFlows" roleURI="http://www.selectabio.com/role/ConsolidatedStatementsofCashFlows">
        <link:definition>1005006 - Statement - Consolidated Statements of Cash Flows</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="DescriptionoftheBusiness" roleURI="http://www.selectabio.com/role/DescriptionoftheBusiness">
        <link:definition>2101101 - Disclosure - Description of the Business</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="DescriptionoftheBusinessDetails" roleURI="http://www.selectabio.com/role/DescriptionoftheBusinessDetails">
        <link:definition>2402401 - Disclosure - Description of the Business (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="SummaryofSignificantAccountingPolicies" roleURI="http://www.selectabio.com/role/SummaryofSignificantAccountingPolicies">
        <link:definition>2103102 - Disclosure - Summary of Significant Accounting Policies</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="SummaryofSignificantAccountingPoliciesPolicies" roleURI="http://www.selectabio.com/role/SummaryofSignificantAccountingPoliciesPolicies">
        <link:definition>2204201 - Disclosure - Summary of Significant Accounting Policies (Policies)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="MarketableSecuritiesandInvestments" roleURI="http://www.selectabio.com/role/MarketableSecuritiesandInvestments">
        <link:definition>2105103 - Disclosure - Marketable Securities and Investments</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="MarketableSecuritiesandInvestmentsTables" roleURI="http://www.selectabio.com/role/MarketableSecuritiesandInvestmentsTables">
        <link:definition>2306301 - Disclosure - Marketable Securities and Investments (Tables)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="MarketableSecuritiesandInvestmentsSummaryofAvailableforSaleMarketableSecuritiesDetails" roleURI="http://www.selectabio.com/role/MarketableSecuritiesandInvestmentsSummaryofAvailableforSaleMarketableSecuritiesDetails">
        <link:definition>2407402 - Disclosure - Marketable Securities and Investments - Summary of Available-for-Sale Marketable Securities (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="MarketableSecuritiesandInvestmentsNarrativeDetails" roleURI="http://www.selectabio.com/role/MarketableSecuritiesandInvestmentsNarrativeDetails">
        <link:definition>2408403 - Disclosure - Marketable Securities and Investments - Narrative (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="NetIncomeLossPerShare" roleURI="http://www.selectabio.com/role/NetIncomeLossPerShare">
        <link:definition>2109104 - Disclosure - Net Income (Loss) Per Share</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="NetIncomeLossPerShareTables" roleURI="http://www.selectabio.com/role/NetIncomeLossPerShareTables">
        <link:definition>2310302 - Disclosure - Net Income (Loss) Per Share (Tables)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="NetIncomeLossPerShareComputationofBasicandDilutedNetIncomeLossPerShareDetails" roleURI="http://www.selectabio.com/role/NetIncomeLossPerShareComputationofBasicandDilutedNetIncomeLossPerShareDetails">
        <link:definition>2411404 - Disclosure - Net Income (Loss) Per Share- Computation of Basic and Diluted Net Income (Loss) Per Share (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="NetIncomeLossPerShareScheduleofPotentialCommonSharesIssuableUponConversionofWarrantsDetails" roleURI="http://www.selectabio.com/role/NetIncomeLossPerShareScheduleofPotentialCommonSharesIssuableUponConversionofWarrantsDetails">
        <link:definition>2412405 - Disclosure - Net Income (Loss) Per Share - Schedule of Potential Common Shares Issuable Upon Conversion of Warrants (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="FairValueMeasurements" roleURI="http://www.selectabio.com/role/FairValueMeasurements">
        <link:definition>2113105 - Disclosure - Fair Value Measurements</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="FairValueMeasurementsTables" roleURI="http://www.selectabio.com/role/FairValueMeasurementsTables">
        <link:definition>2314303 - Disclosure - Fair Value Measurements (Tables)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="FairValueMeasurementsSummaryofAssetsMeasuredatFairValueonaRecurringBasisDetails" roleURI="http://www.selectabio.com/role/FairValueMeasurementsSummaryofAssetsMeasuredatFairValueonaRecurringBasisDetails">
        <link:definition>2415406 - Disclosure - Fair Value Measurements - Summary of Assets Measured at Fair Value on a Recurring Basis (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="FairValueMeasurementsNarrativeDetails" roleURI="http://www.selectabio.com/role/FairValueMeasurementsNarrativeDetails">
        <link:definition>2416407 - Disclosure - Fair Value Measurements - Narrative (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="FairValueMeasurementsScheduleofCashandCashEquivalentsDetails" roleURI="http://www.selectabio.com/role/FairValueMeasurementsScheduleofCashandCashEquivalentsDetails">
        <link:definition>2417408 - Disclosure - Fair Value Measurements - Schedule of Cash and Cash Equivalents (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="FairValueMeasurementsAssumptionsusedtorecordthefairvalueofthewarrantsDetails" roleURI="http://www.selectabio.com/role/FairValueMeasurementsAssumptionsusedtorecordthefairvalueofthewarrantsDetails">
        <link:definition>2418409 - Disclosure - Fair Value Measurements - Assumptions used to record the fair value of the warrants (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="FairValueMeasurementsChangeinthewarrantliabilitiesDetails" roleURI="http://www.selectabio.com/role/FairValueMeasurementsChangeinthewarrantliabilitiesDetails">
        <link:definition>2419410 - Disclosure - Fair Value Measurements - Change in the warrant liabilities (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="PropertyandEquipment" roleURI="http://www.selectabio.com/role/PropertyandEquipment">
        <link:definition>2120106 - Disclosure - Property and Equipment</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="PropertyandEquipmentTables" roleURI="http://www.selectabio.com/role/PropertyandEquipmentTables">
        <link:definition>2321304 - Disclosure - Property and Equipment (Tables)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="PropertyandEquipmentScheduleofPropertyandEquipmentDetails" roleURI="http://www.selectabio.com/role/PropertyandEquipmentScheduleofPropertyandEquipmentDetails">
        <link:definition>2422411 - Disclosure - Property and Equipment - Schedule of Property and Equipment (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="PropertyandEquipmentNarrativeDetails" roleURI="http://www.selectabio.com/role/PropertyandEquipmentNarrativeDetails">
        <link:definition>2423412 - Disclosure - Property and Equipment - Narrative (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="AccruedExpenses" roleURI="http://www.selectabio.com/role/AccruedExpenses">
        <link:definition>2124107 - Disclosure - Accrued Expenses</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="AccruedExpensesTables" roleURI="http://www.selectabio.com/role/AccruedExpensesTables">
        <link:definition>2325305 - Disclosure - Accrued Expenses (Tables)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="AccruedExpensesDetails" roleURI="http://www.selectabio.com/role/AccruedExpensesDetails">
        <link:definition>2426413 - Disclosure - Accrued Expenses (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="AccruedExpensesNarrativeDetails" roleURI="http://www.selectabio.com/role/AccruedExpensesNarrativeDetails">
        <link:definition>2427414 - Disclosure - Accrued Expenses - Narrative (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="Leases" roleURI="http://www.selectabio.com/role/Leases">
        <link:definition>2128108 - Disclosure - Leases</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="LeasesTables" roleURI="http://www.selectabio.com/role/LeasesTables">
        <link:definition>2329306 - Disclosure - Leases (Tables)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="LeasesComponentsofleasecostsDetails" roleURI="http://www.selectabio.com/role/LeasesComponentsofleasecostsDetails">
        <link:definition>2430415 - Disclosure - Leases - Components of lease costs (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="LeasesOperatingLeaseLiabilityMaturityDetails" roleURI="http://www.selectabio.com/role/LeasesOperatingLeaseLiabilityMaturityDetails">
        <link:definition>2431416 - Disclosure - Leases - Operating Lease, Liability, Maturity (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="LeasesOperatingLeaseLiabilityMaturityDetails_1" roleURI="http://www.selectabio.com/role/LeasesOperatingLeaseLiabilityMaturityDetails_1">
        <link:definition>2431416 - Disclosure - Leases - Operating Lease, Liability, Maturity (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="LeasesOtherInformationDetails" roleURI="http://www.selectabio.com/role/LeasesOtherInformationDetails">
        <link:definition>2432417 - Disclosure - Leases - Other Information (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="LeasesLeaseTermandDiscountRateDetails" roleURI="http://www.selectabio.com/role/LeasesLeaseTermandDiscountRateDetails">
        <link:definition>2433418 - Disclosure - Leases - Lease Term and Discount Rate (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="Debt" roleURI="http://www.selectabio.com/role/Debt">
        <link:definition>2134109 - Disclosure - Debt</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="DebtTables" roleURI="http://www.selectabio.com/role/DebtTables">
        <link:definition>2335307 - Disclosure - Debt (Tables)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="DebtTermLoansNarrativeDetails" roleURI="http://www.selectabio.com/role/DebtTermLoansNarrativeDetails">
        <link:definition>2436419 - Disclosure - Debt - Term Loans (Narrative) (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="DebtTermLoanAndUnamortizedDebtDiscountBalancesDetails" roleURI="http://www.selectabio.com/role/DebtTermLoanAndUnamortizedDebtDiscountBalancesDetails">
        <link:definition>2437420 - Disclosure - Debt - Term Loan And Unamortized Debt Discount Balances (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="DebtScheduleofFutureMinimumPaymentsontheTermLoansDetails" roleURI="http://www.selectabio.com/role/DebtScheduleofFutureMinimumPaymentsontheTermLoansDetails">
        <link:definition>2438421 - Disclosure - Debt - Schedule of Future Minimum Payments on the Term Loans (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="Equity" roleURI="http://www.selectabio.com/role/Equity">
        <link:definition>2139110 - Disclosure - Equity</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="EquityTables" roleURI="http://www.selectabio.com/role/EquityTables">
        <link:definition>2340308 - Disclosure - Equity (Tables)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="EquityNarrativeDetails" roleURI="http://www.selectabio.com/role/EquityNarrativeDetails">
        <link:definition>2441422 - Disclosure - Equity - Narrative (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="EquityWarrantyActivityDetails" roleURI="http://www.selectabio.com/role/EquityWarrantyActivityDetails">
        <link:definition>2442423 - Disclosure - Equity - Warranty Activity (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="EquityScheduleofAuthorizedSharesofCommonStockforFutureIssuanceDetails" roleURI="http://www.selectabio.com/role/EquityScheduleofAuthorizedSharesofCommonStockforFutureIssuanceDetails">
        <link:definition>2443424 - Disclosure - Equity - Schedule of Authorized Shares of Common Stock for Future Issuance (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="StockIncentivePlans" roleURI="http://www.selectabio.com/role/StockIncentivePlans">
        <link:definition>2144111 - Disclosure - Stock Incentive Plans</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="StockIncentivePlansTables" roleURI="http://www.selectabio.com/role/StockIncentivePlansTables">
        <link:definition>2345309 - Disclosure - Stock Incentive Plans (Tables)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="StockIncentivePlansNarrativeDetails" roleURI="http://www.selectabio.com/role/StockIncentivePlansNarrativeDetails">
        <link:definition>2446425 - Disclosure - Stock Incentive Plans - Narrative (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="StockIncentivePlansScheduleofStockBasedCompensationExpenseRelatedtoStockOptionsandRestrictedCommonStockDetails" roleURI="http://www.selectabio.com/role/StockIncentivePlansScheduleofStockBasedCompensationExpenseRelatedtoStockOptionsandRestrictedCommonStockDetails">
        <link:definition>2447426 - Disclosure - Stock Incentive Plans - Schedule of Stock-Based Compensation Expense Related to Stock Options and Restricted Common Stock (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="StockIncentivePlansScheduleofAssumptionsDetails" roleURI="http://www.selectabio.com/role/StockIncentivePlansScheduleofAssumptionsDetails">
        <link:definition>2448427 - Disclosure - Stock Incentive Plans - Schedule of Assumptions (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="StockIncentivePlansSummaryofOptionsActivityDetails" roleURI="http://www.selectabio.com/role/StockIncentivePlansSummaryofOptionsActivityDetails">
        <link:definition>2449428 - Disclosure - Stock Incentive Plans - Summary of Options Activity (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="StockIncentivePlansRestrictedStockUnitsDetails" roleURI="http://www.selectabio.com/role/StockIncentivePlansRestrictedStockUnitsDetails">
        <link:definition>2450429 - Disclosure - Stock Incentive Plans - Restricted Stock Units (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="StockIncentivePlansEmployeeStockPurchasePlanNarrativeDetails" roleURI="http://www.selectabio.com/role/StockIncentivePlansEmployeeStockPurchasePlanNarrativeDetails">
        <link:definition>2451430 - Disclosure - Stock Incentive Plans - Employee Stock Purchase Plan (Narrative) (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="RevenueArrangements" roleURI="http://www.selectabio.com/role/RevenueArrangements">
        <link:definition>2152112 - Disclosure - Revenue Arrangements</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="RevenueArrangementsTables" roleURI="http://www.selectabio.com/role/RevenueArrangementsTables">
        <link:definition>2353310 - Disclosure - Revenue Arrangements (Tables)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="RevenueArrangementsNarrativeDetails" roleURI="http://www.selectabio.com/role/RevenueArrangementsNarrativeDetails">
        <link:definition>2454431 - Disclosure - Revenue Arrangements - Narrative (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="RevenueArrangementsScheduleofChangesinContractLiabilitiesDetails" roleURI="http://www.selectabio.com/role/RevenueArrangementsScheduleofChangesinContractLiabilitiesDetails">
        <link:definition>2455432 - Disclosure - Revenue Arrangements - Schedule of Changes in Contract Liabilities (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="RelatedpartyTransactions" roleURI="http://www.selectabio.com/role/RelatedpartyTransactions">
        <link:definition>2156113 - Disclosure - Related-party Transactions</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="RelatedPartyTransactionsTables" roleURI="http://www.selectabio.com/role/RelatedPartyTransactionsTables">
        <link:definition>2357311 - Disclosure - Related-Party Transactions - (Tables)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="RelatedPartyTransactionsNumberofSharesofCommonStockPurchasedDetails" roleURI="http://www.selectabio.com/role/RelatedPartyTransactionsNumberofSharesofCommonStockPurchasedDetails">
        <link:definition>2458433 - Disclosure - Related-Party Transactions - Number of Shares of Common Stock Purchased (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="RelatedpartyTransactionsNarrativeDetails" roleURI="http://www.selectabio.com/role/RelatedpartyTransactionsNarrativeDetails">
        <link:definition>2459434 - Disclosure - Related-party Transactions - Narrative (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="CollaborationandLicenseAgreements" roleURI="http://www.selectabio.com/role/CollaborationandLicenseAgreements">
        <link:definition>2160114 - Disclosure - Collaboration and License Agreements</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="CollaborationandLicenseAgreementsGinkgoAgreementDetails" roleURI="http://www.selectabio.com/role/CollaborationandLicenseAgreementsGinkgoAgreementDetails">
        <link:definition>2461435 - Disclosure - Collaboration and License Agreements - Ginkgo Agreement (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="CollaborationandLicenseAgreementsCyrusBiotechnologyIncNarrativeDetails" roleURI="http://www.selectabio.com/role/CollaborationandLicenseAgreementsCyrusBiotechnologyIncNarrativeDetails">
        <link:definition>2462436 - Disclosure - Collaboration and License Agreements - Cyrus Biotechnology, Inc. (Narrative) (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="CollaborationandLicenseAgreementsAskBioNarrativeDetails" roleURI="http://www.selectabio.com/role/CollaborationandLicenseAgreementsAskBioNarrativeDetails">
        <link:definition>2463437 - Disclosure - Collaboration and License Agreements - AskBio (Narrative) (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="CollaborationandLicenseAgreementsMITNarrativeDetails" roleURI="http://www.selectabio.com/role/CollaborationandLicenseAgreementsMITNarrativeDetails">
        <link:definition>2464438 - Disclosure - Collaboration and License Agreements - MIT (Narrative) (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="CollaborationandLicenseAgreementsShenyangSunshinePharmaceuticalCoLtdNarrativeDetails" roleURI="http://www.selectabio.com/role/CollaborationandLicenseAgreementsShenyangSunshinePharmaceuticalCoLtdNarrativeDetails">
        <link:definition>2465439 - Disclosure - Collaboration and License Agreements - Shenyang Sunshine Pharmaceutical Co., Ltd (Narrative) (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="IncomeTaxes" roleURI="http://www.selectabio.com/role/IncomeTaxes">
        <link:definition>2166115 - Disclosure - Income Taxes</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="IncomeTaxesNarrativeDetails" roleURI="http://www.selectabio.com/role/IncomeTaxesNarrativeDetails">
        <link:definition>2467440 - Disclosure - Income Taxes - Narrative (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="DefinedContributionPlan" roleURI="http://www.selectabio.com/role/DefinedContributionPlan">
        <link:definition>2168116 - Disclosure - Defined Contribution Plan</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="DefinedContributionPlanNarrativeDetails" roleURI="http://www.selectabio.com/role/DefinedContributionPlanNarrativeDetails">
        <link:definition>2469441 - Disclosure - Defined Contribution Plan - Narrative (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="CommitmentsandContingencies" roleURI="http://www.selectabio.com/role/CommitmentsandContingencies">
        <link:definition>2170117 - Disclosure - Commitments and Contingencies</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="CommitmentsandContingenciesDetails" roleURI="http://www.selectabio.com/role/CommitmentsandContingenciesDetails">
        <link:definition>2471442 - Disclosure - Commitments and Contingencies (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="SubsequentEvents" roleURI="http://www.selectabio.com/role/SubsequentEvents">
        <link:definition>2172118 - Disclosure - Subsequent Events</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
    </xs:appinfo>
  </xs:annotation>
  <xs:element id="selb_ProceedsFromIssuanceOfCommonStockAndWarrants" abstract="false" name="ProceedsFromIssuanceOfCommonStockAndWarrants" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="selb_LicenseAndOptionAgreementExtensionTerm" abstract="false" name="LicenseAndOptionAgreementExtensionTerm" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:durationItemType"/>
  <xs:element id="selb_LicenseAndOptionAgreementMilestoneReceivable" abstract="false" name="LicenseAndOptionAgreementMilestoneReceivable" nillable="true" xbrli:periodType="instant" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="selb_A2019WarrantsMember" abstract="true" name="A2019WarrantsMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="selb_ContractwithCustomerLiabilityDeferredRevenueRevenueRecognized" abstract="false" name="ContractwithCustomerLiabilityDeferredRevenueRevenueRecognized" nillable="true" xbrli:periodType="duration" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="selb_DefinedContributionPlanEmployersMatchingContributionVestingPeriod" abstract="false" name="DefinedContributionPlanEmployersMatchingContributionVestingPeriod" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:durationItemType"/>
  <xs:element id="selb_LesseeOperatingLeaseLiabilityToBePaidAfterYearFour" abstract="false" name="LesseeOperatingLeaseLiabilityToBePaidAfterYearFour" nillable="true" xbrli:periodType="instant" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="selb_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValueAbstract" abstract="true" name="ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValueAbstract" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:stringItemType"/>
  <xs:element id="selb_LicenseAndOptionAgreementExtensionReceivable" abstract="false" name="LicenseAndOptionAgreementExtensionReceivable" nillable="true" xbrli:periodType="instant" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="selb_CyrusBiotechnologyIncMember" abstract="true" name="CyrusBiotechnologyIncMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="selb_LicenseAndOptionAgreementInitialUpFrontCashPayment" abstract="false" name="LicenseAndOptionAgreementInitialUpFrontCashPayment" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="selb_LicenseAndOptionAgreementExecutionFeePayment" abstract="false" name="LicenseAndOptionAgreementExecutionFeePayment" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="selb_CollaborationAndLicenseAgreementsTextBlock" abstract="false" name="CollaborationAndLicenseAgreementsTextBlock" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:textBlockItemType"/>
  <xs:element id="selb_SOBIPurchaseAgreementMember" abstract="true" name="SOBIPurchaseAgreementMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="selb_A2022WarrantsMember" abstract="true" name="A2022WarrantsMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="selb_EmploymentInducementIncentiveAwardPlanMember" abstract="true" name="EmploymentInducementIncentiveAwardPlanMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="selb_LicenseAndOptionAgreementSalesMilestonePaymentsThatMayBeReceived" abstract="false" name="LicenseAndOptionAgreementSalesMilestonePaymentsThatMayBeReceived" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="selb_StockIncentivePlan2016Member" abstract="false" name="StockIncentivePlan2016Member" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="selb_LicenseAndOptionAgreementClinicalAndCommercialMilestonePaymentExpected" abstract="false" name="LicenseAndOptionAgreementClinicalAndCommercialMilestonePaymentExpected" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="selb_ContractwithCustomerLiabilityDeferredRevenue" abstract="false" name="ContractwithCustomerLiabilityDeferredRevenue" nillable="true" xbrli:periodType="instant" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="selb_CollaborativeArrangementPaymentObligation" abstract="false" name="CollaborativeArrangementPaymentObligation" nillable="true" xbrli:periodType="duration" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="selb_AccruedCollaborationandLicensing" abstract="false" name="AccruedCollaborationandLicensing" nillable="true" xbrli:periodType="instant" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="selb_ChangeInContractWithCustomerLiabilityRollForward" abstract="true" name="ChangeInContractWithCustomerLiabilityRollForward" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:stringItemType"/>
  <xs:element id="selb_LicenseAndOptionAgreementUpfrontCashPayment" abstract="false" name="LicenseAndOptionAgreementUpfrontCashPayment" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="selb_ExclusivePatentLicenseAgreementWithMassachusettsInstituteOfTechnologyMember" abstract="false" name="ExclusivePatentLicenseAgreementWithMassachusettsInstituteOfTechnologyMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="selb_DebtInstrumentTerminationFee" abstract="false" name="DebtInstrumentTerminationFee" nillable="true" xbrli:periodType="instant" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="selb_EquitySecuritiesStockPurchaseAgreementSharesPurchased" abstract="false" name="EquitySecuritiesStockPurchaseAgreementSharesPurchased" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:sharesItemType"/>
  <xs:element id="selb_LicenseAndOptionAgreementMilestonePayments" abstract="false" name="LicenseAndOptionAgreementMilestonePayments" nillable="true" xbrli:periodType="duration" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="selb_AccruedPatentFeesCurrent" abstract="false" name="AccruedPatentFeesCurrent" nillable="true" xbrli:periodType="instant" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="selb_AskBioLicenseMember" abstract="true" name="AskBioLicenseMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="selb_LicenseAndOptionAgreementFutureAdditionalPaymentsExpected" abstract="false" name="LicenseAndOptionAgreementFutureAdditionalPaymentsExpected" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="selb_ContractwithCustomerLiabilityDeferredRevenueIncreaseFromCashReceipts" abstract="false" name="ContractwithCustomerLiabilityDeferredRevenueIncreaseFromCashReceipts" nillable="true" xbrli:periodType="duration" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="selb_TASPartnersLLCMember" abstract="true" name="TASPartnersLLCMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="selb_FounderMember" abstract="false" name="FounderMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="selb_ContractwithCustomerLiabilityIncreaseFromCashReceipts" abstract="false" name="ContractwithCustomerLiabilityIncreaseFromCashReceipts" nillable="true" xbrli:periodType="duration" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="selb_NonEmployeeStockOptionMember" abstract="true" name="NonEmployeeStockOptionMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="selb_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTermAbstract" abstract="true" name="ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTermAbstract" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:stringItemType"/>
  <xs:element id="selb_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsWeightedAverageFairValue" abstract="false" name="ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsWeightedAverageFairValue" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:perShareItemType"/>
  <xs:element id="selb_ShareBasedCompensationArrangementByShareBasedPaymentAwardForfeituresRate" abstract="false" name="ShareBasedCompensationArrangementByShareBasedPaymentAwardForfeituresRate" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:percentItemType"/>
  <xs:element id="selb_EligiblePeriodToReceiveRoyalties" abstract="false" name="EligiblePeriodToReceiveRoyalties" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:durationItemType"/>
  <xs:element id="selb_CollaborativeArrangementCostSharePercentage" abstract="false" name="CollaborativeArrangementCostSharePercentage" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:percentItemType"/>
  <xs:element id="selb_AtTheMarketOfferingMember" abstract="true" name="AtTheMarketOfferingMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="selb_RevenueNumberOfPerformanceObligations" abstract="false" name="RevenueNumberOfPerformanceObligations" nillable="true" xbrli:periodType="instant" substitutionGroup="xbrli:item" type="xbrli:integerItemType"/>
  <xs:element id="selb_A2020TermLoansMember" abstract="true" name="A2020TermLoansMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="selb_TakedaAgreementMember" abstract="true" name="TakedaAgreementMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="selb_WarrantExercisePriceCanceled" abstract="false" name="WarrantExercisePriceCanceled" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:perShareItemType"/>
  <xs:element id="selb_LicenseAndOptionAgreementDevelopmentMilestone" abstract="false" name="LicenseAndOptionAgreementDevelopmentMilestone" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="selb_StockIssuedDuringPeriodSharesWarrantsExercised" abstract="false" name="StockIssuedDuringPeriodSharesWarrantsExercised" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:sharesItemType"/>
  <xs:element id="selb_GinkgoAgreementMember" abstract="true" name="GinkgoAgreementMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="selb_LicenseAndOptionAgreementWrittenDayNoticePeriodBeforeCancellation" abstract="false" name="LicenseAndOptionAgreementWrittenDayNoticePeriodBeforeCancellation" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:durationItemType"/>
  <xs:element id="selb_SareptaTherapeuticsInc.Member" abstract="true" name="SareptaTherapeuticsInc.Member" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="selb_LicenseAndOptionAgreementNumberOfAmendments" abstract="false" name="LicenseAndOptionAgreementNumberOfAmendments" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:integerItemType"/>
  <xs:element id="selb_StockIssuedDuringPeriodSharesNewIssuesUnderLicenseAgreement" abstract="false" name="StockIssuedDuringPeriodSharesNewIssuesUnderLicenseAgreement" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:sharesItemType"/>
  <xs:element id="selb_LicenseAndOptionAgreementCalculatedPremiumPaidForEquityInvestmentUnderPurchaseAgreement" abstract="false" name="LicenseAndOptionAgreementCalculatedPremiumPaidForEquityInvestmentUnderPurchaseAgreement" nillable="true" xbrli:periodType="duration" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="selb_StockIssuedDuringPeriodSharesWarrantsCanceled" abstract="false" name="StockIssuedDuringPeriodSharesWarrantsCanceled" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:sharesItemType"/>
  <xs:element id="selb_AccruedAuditFeesCurrent" abstract="false" name="AccruedAuditFeesCurrent" nillable="true" xbrli:periodType="instant" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="selb_LaboratoryEquipmentMember" abstract="false" name="LaboratoryEquipmentMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="selb_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedOutstandingWeightedAverageRemainingContractualTerm" abstract="false" name="SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedOutstandingWeightedAverageRemainingContractualTerm" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:durationItemType"/>
  <xs:element id="selb_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedOutstandingIntrinsicValue" abstract="false" name="SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedOutstandingIntrinsicValue" nillable="true" xbrli:periodType="instant" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="selb_EquityOfferingCostsInAccruedLiabilities" abstract="false" name="EquityOfferingCostsInAccruedLiabilities" nillable="true" xbrli:periodType="duration" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="selb_AccruedResearchAndDevelopmentCostsCurrent" abstract="false" name="AccruedResearchAndDevelopmentCostsCurrent" nillable="true" xbrli:periodType="instant" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="selb_ResearchAndDevelopmentMember" abstract="true" name="ResearchAndDevelopmentMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="selb_LicenseAndOptionAgreementOptionTerm" abstract="false" name="LicenseAndOptionAgreementOptionTerm" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:durationItemType"/>
  <xs:element id="selb_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRightsRollForward" abstract="true" name="ClassOfWarrantOrRightExercisePriceOfWarrantsOrRightsRollForward" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:stringItemType"/>
  <xs:element id="selb_EquitySecuritiesStockPurchaseAgreementParValuePerShare" abstract="false" name="EquitySecuritiesStockPurchaseAgreementParValuePerShare" nillable="true" xbrli:periodType="instant" substitutionGroup="xbrli:item" type="dtr-types:perShareItemType"/>
  <xs:element id="selb_StockIssuedDuringPeriodValueNewIssuesUnderLicenseAgreement" abstract="false" name="StockIssuedDuringPeriodValueNewIssuesUnderLicenseAgreement" nillable="true" xbrli:periodType="duration" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="selb_EmployeeStockPurchasePlan2016Member" abstract="false" name="EmployeeStockPurchasePlan2016Member" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="selb_SharebasedCompensationArrangementbySharebasedPaymentAwardOptionsVestedWeightedAverageExercisePrice" abstract="false" name="SharebasedCompensationArrangementbySharebasedPaymentAwardOptionsVestedWeightedAverageExercisePrice" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:perShareItemType"/>
  <xs:element id="selb_AggregateAmountOfUpfrontAndMilestoneBasedPaymentsPaid" abstract="false" name="AggregateAmountOfUpfrontAndMilestoneBasedPaymentsPaid" nillable="true" xbrli:periodType="instant" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="selb_UnderwritingAgreementMember" abstract="true" name="UnderwritingAgreementMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="selb_A2008Plan2016PlanAnd2018InducementIncentiveAwardPlanMember" abstract="true" name="A2008Plan2016PlanAnd2018InducementIncentiveAwardPlanMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="selb_AccruedInterestCurrent" abstract="false" name="AccruedInterestCurrent" nillable="true" xbrli:periodType="instant" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="selb_SeriesBPreferredStockPurchaseAgreementMember" abstract="true" name="SeriesBPreferredStockPurchaseAgreementMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="selb_LicenseAgreementWithShenyangSunshinePharmaceuticalCompanyLimitedMember" abstract="false" name="LicenseAgreementWithShenyangSunshinePharmaceuticalCompanyLimitedMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="selb_ClassOfWarrantOrRightRollForward" abstract="true" name="ClassOfWarrantOrRightRollForward" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:stringItemType"/>
  <xs:element id="selb_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsAndDebtSecuritiesAvailableForSale" abstract="false" name="CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsAndDebtSecuritiesAvailableForSale" nillable="true" xbrli:periodType="instant" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="selb_AggregateAmountForFuturePaymentsUponAchievementOfClinicalAndRegulatoryApprovalMilestonesForProductsContainingImmTORPlatform" abstract="false" name="AggregateAmountForFuturePaymentsUponAchievementOfClinicalAndRegulatoryApprovalMilestonesForProductsContainingImmTORPlatform" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="selb_RestrictedStockUnitsUnvestedMember" abstract="true" name="RestrictedStockUnitsUnvestedMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="selb_EquitySecuritiesStockPurchaseAgreementPurchasePricePerShare" abstract="false" name="EquitySecuritiesStockPurchaseAgreementPurchasePricePerShare" nillable="true" xbrli:periodType="instant" substitutionGroup="xbrli:item" type="dtr-types:perShareItemType"/>
  <xs:element id="selb_FairValueMeasurementwithUnobservableInputsReconciliationRecurringBasisLiabilityExercisesInPeriod" abstract="false" name="FairValueMeasurementwithUnobservableInputsReconciliationRecurringBasisLiabilityExercisesInPeriod" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="selb_LicenseAgreementForPompeDiseaseMember" abstract="true" name="LicenseAgreementForPompeDiseaseMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="selb_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrantIncreaseDecrease" abstract="false" name="ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrantIncreaseDecrease" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:sharesItemType"/>
  <xs:element id="selb_UnderwritingAgreement2022WarrantsMember" abstract="true" name="UnderwritingAgreement2022WarrantsMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="selb_StockIssuedDuringPeriodSharesWarrantsIssued" abstract="false" name="StockIssuedDuringPeriodSharesWarrantsIssued" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:sharesItemType"/>
  <xs:element id="selb_LicenseAndOptionAgreementContractualPaymentsMade" abstract="false" name="LicenseAndOptionAgreementContractualPaymentsMade" nillable="true" xbrli:periodType="duration" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
</xs:schema>
