Medical Microbiology

tionpathways,TandBcellsdifferwithrespecttotheirfunctions,receptors,

andsurfacemarkers.Theymanifestcontrastingresponsepatternstocyto-

kines,anddisplayamarkedpreferencetooccupydifferentcompartments

oflymphoidorgans.TandBcellscommunicatewitheachother,andwith

othercelltypes,bymeansofadhesionandaccessorymolecules(CDantigens,

seeTable2. 13 ,p. 13 7)orinresponsetosolublefactors,suchascytokines,

whichbindtospecificreceptorsandinducetheactivationofintracellularsig-

nalingpathways.Theantigen-dependentdifferentiationprocesseswhich

leadstoTandBcellspecialization,takesplacewithinthesecondarylym-

phoidorganswherelymphocytescomeintocontactwithantigens.Asa

generalrulethesecondarylymphoidorganscontainonlymatureTandB

cells,andcompriseencapsulatedorganssuchasthelymphnodesand

spleen,ornon-encapsulatedstructureswhichcontainlymphocytesand

areassociatedwiththeskin,mucosa,gut,orbronchus(i.e.SALT,MALT,

GALT,andBALT).Together,theprimaryandsecondarylymphoidorgansac-

countforapproximately 1 – 2 %ofbodyweight.

TheB-CellSystem

&Blymphocytesproduceantibodiesintwoforms;amembrane-bound

formandasecretedform.Membrane-boundantibodyformstheB-cellanti-

genreceptor.Followingantigenstimulation,Blymphocytesdifferentiateinto

plasmacells,whichsecreteantibodiesexhibitingthesameantigenspecifi-

cityastheB-cellreceptor.Thissystemischaracterizedashumoralimmu-

nity,duetothisreleaseofreceptorsintothe“humoral”systemwhich

constitutes vascular contents andmucous environments.The humoral

48 2 BasicPrinciplesofImmunology

Table2. 1 DistributionofLymphocyteSubpopulationsandAPCsinVariousOr-

gans(%ofAllMononuclearCells)

B T NK,LAK,ADCC APC

Peripheralblood 10 – 15 70 – 80 5 – 10 < 1

Lymph 5 95 – 100??

Thoracicduct 5 – 10 90 – 95??

Thymus 1 95 – 100? 0

Bonemarrow 15 – 20 10 – 15? 0

Spleen 40 – 50 40 – 60 20 – 30 1

Lymphnodes,tonsils,etc. 20 – 30 70 – 80 5 – 8 1

NK:naturalkillercells;LAK:lymphokine-activatedkillercells,ADCC:antibody-depen-

dentcellulartoxicity,APC:antigen-presentingcells

2

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Kayser, Medical Microbiology © 2005 Thieme

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