LE 43-10Lymphoidstem cellBone marrowThymusT cellBlood, lymph, and lymphoid tissues(lymph nodes, spleen, and others)B cellLE 43-8Antigen-bindingsiteDisulfidebridgeLightchainVVCCCCVVAntigen-binding siteVariable regionsConstant regionsTransmembraneregionAntigen-binding siteCVCVPlasmamembraneT cellβ chainα chainDisulfide bridgeCytoplasm of T cellA T cell receptor consists of one αchain and one β chain linked by adisulfide bridge.A B cell receptor consists of two identical heavychains and two identical light chains linked byseveral disulfide bridges.Heavy chainsCytoplasm of B cellB cellFig. 43-9a Antigen-bindingsite Antigen-binding siteDisulfidebridgeVariableregionsConstantregionsTransmembraneregionPlasmamembraneLightchainHeavy chainsCytoplasm of B cell(a) B cell receptorB cellVVCCVVC CFig. 43-9b Antigen-bindingsite VariableregionsConstantregionsTransmembraneregionPlasmamembraneT cellα chainβ chainDisulfide bridgeCytoplasm of T cell(b) T cell receptorC CVVFig. 43-11AntigenTop view: binding surfaceexposed to antigen receptorsPlasmamembrane ofinfected cellAntigenClass I MHCmoleculeLE 43-9Antigen-presentingcellAntigenfragmentClass II MHCmoleculeT cellreceptorHelper T cellAntigenfragmentClass I MHCmoleculeT cellreceptorCytotoxic T cellInfected cellMicrobeFig. 43-18-1Cytotoxic T cellPerforinGranzymesTCRCD8Class I MHCmoleculeTargetcellPeptideantigenFig. 43-18-2Cytotoxic T cellPerforinGranzymesTCRCD8Class I MHCmoleculeTargetcellPeptideantigenPoreFig. 43-18-3Cytotoxic T cellPerforinGranzymesTCRCD8Class I MHCmoleculeTargetcellPeptideantigenPoreReleased cytotoxic T cellDying target cellFig. 43-17Antigen-presentingcellPeptide antigenCell-mediatedimmunity (attack oninfected cells)Class II MHC moleculeCD4TCR (T cell receptor)Helper T cellHumoralimmunity (secretion ofantibodies byplasma cells)Cytotoxic T cellCytokinesB cellBacterium++ ++Fig. 43-19-1Antigen-presenting cellBacteriumPeptideantigenClass II MHCmoleculeTCRCD4Helper T cellFig. 43-19-2Antigen-presenting cellBacteriumPeptideantigenClass II MHCmoleculeTCRCD4Helper T cellB cellActivatedhelper T cellCytokines+Fig. 43-19-3Antigen-presenting cellBacteriumPeptideantigenClass II MHCmoleculeTCRCD4Helper T cellB cellActivatedhelper T cellCytokines+SecretedantibodymoleculesClone of memoryB cellsClone of plasma cellsFig. 43-16Humoral (antibody-mediated) immune response B cellPlasma cellsCell-mediated immune response KeyStimulatesGives rise to+++++++Memory B cellsAntigen (1st exposure)Engulfed byAntigen-presenting cellMemoryHelper T cellsHelper T cell Cytotoxic T cellMemoryCytotoxic T cellsActiveCytotoxic T cellsAntigen (2nd exposure)SecretedantibodiesDefend against extracellular pathogens by binding to antigens,thereby neutralizing pathogens or making them better targetsfor phagocytes and complement proteins. Defend against intracellular pathogensand cancer by binding to and lysing theinfected cells or cancer cells.++ +Fig. 43-16aKeyStimulatesGives rise to+MemoryHelper T cellsAntigen-presenting cellHelper T cellEngulfed byAntigen (1st exposure)++++++Defend against extracellular pathogensMemoryB cellsAntigen (2nd exposure)Plasma cellsB cellSecretedantibodiesHumoral (antibody-mediated) immune responseFig. 43-16bCell-mediated immune responseDefend against intracellular pathogensActiveCytotoxic T cellsMemoryCytotoxic T cellsMemoryHelper T cellsAntigen-presenting cellAntigen (2nd exposure)Helper T cellEngulfed byAntigen (1st exposure)Cytotoxic T cellKeyStimulatesGives rise to+++++
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