Medical Microbiology

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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.

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Kayser, Medical Microbiology © 2005 Thieme
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