Medical Microbiology

forTcellactivationtooccur,antigen-transportingAPCsmustfirstreachthe

organizedsecondarylymphoidorgans(Fig.2. 12 ),sincepropercontactbetween

lymphocytesandAPCscanonlytakeplacewithinthesehighlyorganizedand

74 2 BasicPrinciplesofImmunology

StructureofLymphNodesandGerminalCenters

Fig.2. 12 Antigencarriedbyantigen-presentingcells(e.g.,Langerhanscellsinthe

skinwhichhavetakenuplocalantigens),orsolubleantigensenterthemarginal

sinusofthelymphnodethroughafferentlymphaticvessels.Inthespleen,blood-

borneantigensaretakenupbyspecializedmacrophagespresentinthemarginal

zone(marginalzonemacrophages,MZM).Eachlymphnodehasitsownarterialand

venousvascularization.TandBcellsmigratefrombloodvessels,throughspecia-

lizedvenuleswithahighendothelium(HEV:highendothelialvenules),intothe

paracortexwhichislargelycomprisedofTcells.ClustersofBcells(so-calledprimary

follicles)arelocatedinthecortex,wherefollowingantigen-stimulation,secondary

follicleswithgerminalcentersdevelop(rightside).ActiveB-cellproliferationoccurs

atthissite.DifferentiationofBcellsbeginswiththeproliferationoftheprimaryB-

cellblastswithinthedarkzoneandinvolvesintensiveinteractionwithantigen-pre-

sentingdendriticcells(DC).Antibodyclassswitchingandsomaticmutationfollows

andtakesplaceinthelightzone,whereFDC(folliculardendriticcells)stimulateB

cellsandstoretheantigen-antibodycomplexesthatfunctiontopreserveantibody

memory.SecondaryB-cellblastsdevelopintoeitherplasmacellsormemoryBcells.

Lymphocytescanonlyleavethelymphnodesthroughefferentlymphvessels.

2

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Kayser, Medical Microbiology © 2005 Thieme

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