T(thymus-derived)lymphocytesexpressantigenreceptorscalledT-cell receptors (TCRs) thatrecognizepeptidefragmentsofproteinantigensthataredisplayedbyMHCmoleculesonthesurfaceofantigen-presentingcells.
B(bonemarrow–derived)lymphocytesexpressmembrane-boundantibodiesthatrecognizeawidevarietyofantigens.Bcellsareactivatedtobecomeplasmacells,whichsecreteantibodies.
Naturalkiller(NK)cellskillcellsthatareinfectedbysomemicrobes,orarestressedanddamagedbeyondrepair.NKcellsexpressinhibitoryreceptorsthatrecognizeMHCmoleculesthatarenormallyexpressedonhealthycells,andarethuspreventedfromkillingnormalcells.
Antigen-presentingcells(APCs)capturemicrobesandotherantigens,transportthemtolymphoidorgans,anddisplaythemforrecognitionbylymphocytes.ThemostefficientAPCsareDCs,whichliveinepitheliaandmosttissues.
Thecellsoftheimmunesystemareorganizedintissues,someofwhicharethesitesofproductionofmaturelymphocytes(thegenerativelymphoidorgans,thebonemarrow,andthymus),andothersarethesitesofimmuneresponses(theperipherallymphoidorgans,includinglymphnodes,spleen,andmucosallymphoidtissues).
Theearlyreactiontomicrobesismediatedbytheinnateimmunesystem,whichisreadytorespondtomicrobes.Componentsoftheinnateimmunesystemincludeepithelialbarriers,phagocytes,NKcells,andplasmaproteins,forexample,ofthecomplementsystem.Innateimmunereactionsareoftenmanifestedasinflammation.Innateimmunity,unlikeadaptiveimmunity,doesnothavefineantigenspecificityormemory.
Thedefensereactionsofadaptiveimmunitydevelopslowly,butaremorepotentandspecialized.
MicrobesandotherforeignantigensarecapturedbyDCsandtransportedtolymphnodes,wheretheantigensarerecognizedbynaïvelymphocytes.Thelymphocytesareactivatedtoproliferateanddifferentiateintoeffectorandmemorycells.
Cell-mediatedimmunityisthereactionofTlymphocytes,designedtocombatcell-associatedmicrobes(e.g.,phagocytosedmicrobesandmicrobesinthecytoplasmofinfectedcells).Humoralimmunityismediatedbyantibodiesandiseffectiveagainstextracellularmicrobes(inthecirculationandmucosallumens).
CD4+ helperTcellshelpBcellstomakeantibodies,activatemacrophagestodestroyingestedmicrobes,stimulaterecruitmentofleukocytes,andregulateallimmuneresponsestoproteinantigens.ThefunctionsofCD4+ Tcellsaremediatedbysecretedproteinscalledcytokines.
CD8+ cytotoxicTlymphocyteskillcellsthatexpressantigensinthecytoplasmthatareseenasforeign(e.g.,virus-infectedandtumorcells)andcanalsoproducecytokines.
Antibodiessecretedbyplasmacellsneutralizemicrobesandblocktheirinfectivity,andpromotethephagocytosisanddestructionofpathogens.Antibodiesalsoconferpassiveimmunitytoneonates.