Medical Microbiology

compartmentalizedorgans.ThecytokinesIFNc,IL-1,IL-2,IL-4,andIL-1 2 play
animportantroleinthisprocess—asdovariousotherfactors.
Duringthesecondphase(Fig.2. 13 ),activatedThelpercellsrecognizethe
sameMHCclassIIpeptidecomplex,butonthesurfaceofaBcell.Priortothis
event,theBcellmusthaverespondedtothesameantigen(byvirtueofitsIg
surfacereceptorrecognizingaconformationalantigenicepitope),theninter-
nalizedtheantigen,processedit,andfinallypresentedpartsofitintheform
oflinearpeptidesboundtoMHCclassIImoleculesonthecellsurfaceforre-
cognitionbytheThelpercell.TheresultingB-Tcellcontactresultsinfurther
interactionsmediatedbyCD4,CD40,andCD28(seeFig.2. 9 ,p.65)—andsends
asignaltotheBcellwhichinitiatestheswitchfromIgMtoIgGorotherIg
classes.Italsoallowsinductionofaprocessofsomaticmutation,andprob-
ablyenhancesthesurvivaloftheBcellintheformofamemoryBcell.

SubpopulationsofTHelperCells......................

Solublesignalingsubstances,cytokines(interleukins),releasedfromThelper
cellscanalsoprovideaninductivestimulusforBcells.Twosubpopulationsof
Thelpercellscanbedifferentiatedbasedonthepatternsofcytokinespro-
duced(Fig.2. 14 ).Infectionsingeneral,butespeciallythosebyintracellular
parasites,inducecytokineproductionbynaturalkiller(NK)cellsinaddition
toastrongThelper 1 (TH1)response.Theresponsebythesecellsischar-
acterizedbyearlygammainterferon(IFNc)production,increasedlevelsof
phagocyteactivity,eliminationoftheantigenbyIFNc-activatedmacro-
phages,productionofIgG2aandothercomplement-binding(opsonizing)
antibodies(seethecomplementsystem,pp.86ff.),andinductionofcytotoxic
T-cellresponses.IL-1 2 functionsasthemostimportantpromoterofTH1cell
functionandadditionallyactsasaninhibitorofTH2cells.
Incontrast,worminfectionsorotherparasiticdiseasesinducetheearly
productionofIL-4,andresultinthedevelopmentofaTH2response.TH2
cells,inturn,recruiteosinophilsandinduceproductionofIgG1andIgEanti-
bodies.Personssufferingfromallergiesandatopicconditionsshowapatho-
logicallyexcessiveTH2responsepotential.IL-4notonlypromotestheTH2
responsebutalsoinhibitsTH1cells.

CytotoxicTCells(CD8+TCells)........................

MatureCD8+Tcellsperformthebiologicallyimportantfunctionoflysingtar-
getcells.TargetcellrecognitioninvolvestheassociationofMHCclassIstruc-
tureswithpeptidesnormallyderivedfromendogenoussources,i.e.,originat-
inginthecellsthemselvesorsynthesizedwithinthembyintracellularpara-
sites.InductionofcytotoxicCD8+Tcellresponseoftendoesnotrequirehelper

ImmuneResponsesandEffectorMechanisms 75

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