Medical Microbiology

structurecontainingidenticalandrepetitivelyarrangeddeterminants—isa
highly efficientandTcell-independent processwhich involves direct
cross-linkingoftheB-cellreceptor.Incontrasttothisprocess,antibodyre-
sponsesagainstmonomericoroligomericantigensarelessefficientand
strictlyrequireTcellhelp,forbothnon-selfandselfantigens.
SomeformsofT-cellresponsesinvolvethereleaseofsolublemediators
(cytokines),whicheffectivelyexpandsthefieldofTcellfunctionbeyondin-
dividualcell-to-cellcontactstoanabilitytoregulatethefunctionoflarge
numbersofsurroundingcells.OtherT-celleffectormechanismsaremediated
inamoreprecisemannerthroughcell-to-cellcontacts.Examplesofthisin-
cludeperforin-dependentcytolysisandinductionofthesignalingpathways
involvedinB-celldifferentiationorIgclassswitching. &

BCells

B-CellEpitopesandB-CellProliferation...............

Burnet’sclonalselectiontheory,formulatedin 1957 ,statesthateveryB-cell
cloneischaracterizedbyanuniqueantigenspecificity,i.e.,itbearsaspecific
antigenreceptor.Accordingly,oncerearrangementoftheIggeneshastaken
place,thecorrespondingproteinwillbeexpressedasasurfacereceptor.At
thesametimefurtherrearrangementisstopped.Thus,onlyoneABS,orone
specificity(oneVHplusVL[eitherjork]),derivedfromasingleallelecanbe
expressedonasinglecell.Thisphenomenoniscalledallelicexclusion.The
bodyfacesalargenumberofdifferentantigensinitslifetime,necessitating
thatacorrespondinglylargenumberofdifferentreceptorspecificities,and
thereforedifferentBcells,mustcontinuouslybeproduced.Whenagiven
antigenentersanorganism,itbindstotheBcellwhichexhibitsthecorrect
receptorspecificityforthatantigen.Onewaytodescribethisprocessistosay
thattheantigenselectsthecorrespondingB-celltypetowhichitmosteffi-
cientlybinds.However,aslongastherespondingBcellsdonotproliferate,
thespecificityoftheresponseisrestrictedtoaverysmallnumberofcells.For
aneffectiveresponse,clonalproliferationoftheresponsiveBcellsmustbe
induced.AfterseveralcelldivisionsBcellsdifferentiateintoplasmacells
whichreleasethespecificreceptorsintothesurroundingsintheformof
solubleantibodies. B-cellstimulationproceedswith,orwithout,Tcell
helpdependingonthestructureandamountofboundantigen.

Antigens.Antigenscanbedividedintotwocategories;thosewhichstimulate
BcellstosecreteantibodieswithoutanyT-cellhelp,andthosewhichrequire
additionalT-cellsignalsforthispurpose.

ImmuneResponsesandEffectorMechanisms 67

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Kayser, Medical Microbiology © 2005 Thieme