Medical Microbiology

&Type 1 T-independentantigens(TI1).Theseincludeparacrystalline,

identicalepitopesarrangedatapproximately 5 – 10 nmintervalsinarepetitive

two-dimensionalpattern(e.g.,proteinsfoundonthesurfaceofviruses,bac-

teria,andparasites);andantigensassociatedwithlipopolysaccharides(LPS).

ThusTI1antigensrepresentstructureswitharepetitivearrangement,which

allowstheengagementofseveralantigenreceptorsatonetimeandresultsin

optimalIgreceptorcross-linking;orstructureswhichresultinsub-optimal

cross-linking,butwhicharecomplementedbyanLPS-mediatedactivation

signal.EithertypeofantigencaninduceBcellactivationintheabsenceof

Tcellhelp.

&Type 2 T-independentantigens(TI2).Theseantigensarelessstringently

arranged,andareusuallyflexibleormobileoncellsurfaces.Theycancross-

linkIgreceptors,buttoalesserextentthanTI1antigens.TI2antigensrequire

asmallamountofindirectlyassociatedThelpinordertoelicitaB-cellre-

sponse(e.g.,hapten-Ficollantigensorviralglycoproteinsoninfectedcellsur-

faces).

&Thelp-dependentantigens.Thesearemonomericoroligomeric(usually

soluble)antigensthatdonotcauseIgcross-linking,andareunabletoinduce

B-cellproliferationontheirown.Inthiscaseanadditionalsignal,providedby

contactwithTcells,isrequiredforB-cellactivation(seealsoB-celltolerance,

p.93ff.).

ReceptorsonthesurfaceofBcellsandsolubleserumantibodiesusuallyre-

cognizeepitopespresentonthesurfaceofnativeantigens.Forproteinanti-

gens,thesegmentsofpolypeptidechainsinvolvedareusuallyspacedfar

apartwhentheproteinisinadenatured,unfolded,state.Aconformational

orstructuralepitopeisnotformedunlesstheantigenispresentinitsnative

configuration.So-calledsequentialorlinearepitopes—formedbycontigu-

oussegmentsofapolypeptidechainandhiddeninsidetheantigen—arelar-

gelyinaccessibletoBcellreceptorsorantibodies,aslongastheantigenmol-

eculeorinfectiousagentretainsitsnativeconfiguration.Theseepitopes

thereforecontributelittletobiologicalprotection.Thespecificroleoflinear

epitopesisaddressedbelowinthecontextofTcell-mediatedimmunity.B

cellsarealsofrequentlyfoundtobecapableofspecificrecognitionofsugar

moleculesonthesurfaceofinfectiousagents,whilstTcellsappeartobein-

capableofrecognizingsuchsugarmolecules.

ProliferationofBcells.Asmentionedabove,contactbetweenone,orafew,

B-cellreceptorsandthecorrelatingantigenicepitopedoesnotinitselfsuffice

fortheinductionofB-cellproliferation.Insteadproliferationrequireseithera

highdegreeofBcellreceptorcross-linkingbyantigen,oradditionalTcell-

mediatedsignals.

Proliferationandtherearrangementofgeneticmaterial—acontinuous

processwhichcanincreasecellularnumbersbyamillion-fold—occasionally

68 2 BasicPrinciplesofImmunology

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Kayser, Medical Microbiology © 2005 Thieme

All rights reserved. Usage subject to terms and conditions of license.