HelperTcellsarefoundintwodistinctcelltypes,Th1andTh2,distinguishedbythecytokinestheyproduceandrespondtoandtheimmuneresponsestheyareinvolvedin.Th1cellsproducepro-inflammatorycytokineslikeIFN-g,TNF-bandIL-2,whileTh2cellsproducethecytokinesIL-4,IL-5,IL-6andIL-13.ThecytokinesproducedbyTh1cellsstimulatethephagocytosisanddestructionofmicrobialpathogenswhileTh2cytokineslikeIL-4generallystimulatetheproductionofantibodiesdirectedtowardlargeextracellularparasites(see“IL-4SignalingPathway”).IL-5stimulateseosinophilresponses,alsopartoftheimmuneresponsetowardlargeextracellularparasites(see“IL-5SignalingPathway”)Th1andTh2areproducedbydifferentiationfromanon-antigenexposedprecursorcelltype,Thp.ExposureofThpcellstoantigenbyantigen-presentingcellsmayresultintheirdifferentiationtoTh0cells,notyetcommittedtobecomeeitherTh1orTh2cells,althoughtheexistenceofTh0cellsiscontroversial.CellscommittedaseitherTh1andTh2cellsarecalledpolarized,whethertheyareeffectorcellsactivelysecretingcytokinesorarememorycells.ThestimulationofThpcellsbyexposuretoantigen-presentingcellsinducestheproliferationofundifferentiatedcells,andtheirexpressionofIL-2andIL-2receptor.ThedifferentiationofTh1cellsandTh2cellsdependsonthecytokinestheyareexposedto.IL-12causesTh1differentiationandblocksTh2cellproduction(see“IL12andStat4DependentSignalingPathwayinTh1Development”pathway),whileIL-4causesTh2differentiationandantagonizesTh1development.IL-18alsoinducesTh1differentiation(See“IL-18signalingpathway”).PolarizedTh1andTh2cellsalsoexpressdistinctsetsofchemokinereceptorsthatfurthermodifytheirhomingandothercellularresponses(see“SelectiveexpressionofchemokinereceptorsduringT-cellpolarization”pathway).ImprovedunderstandingofTh1andTh2differentiationwillimproveouroverallunderstandingoftheimmunesystem,itsresponsetoinfectionandaberrantresponsesthatleadtodisease.

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