Medical Microbiology

cesses(Table2. 1 ,Fig.2. 2 ).Theantigen-independentphaseoflymphocyte
differentiationtakes placeintheso-calledprimary lymphoidorgans:
TlymphocytesmatureinthethymusandBlymphocytesinthebursafabricI
(inbirds).Althoughmammalshavenobursa,thetermBlymphocytes(or
Bcells)hasbeenretainedtodistinguishthesecells,withtheirclearlydistinct
functionsandmaturationinthebonemarrow,fromTlymphocytes,which
matureinthethymus(Table2. 1 ).Bcellsmatureinthefetalliveraswell
asinfetalandadultbonemarrow.Inadditiontotheirdivergentdifferentia-

TheImmunologicalApparatus 47

MaturationofBandTcells

CD4

CD8

αβ

Primary (central) lymphoid organs

Antigen-independent

Thymic cortex

B cells

Stem cell

T cells

Progenitor

B (pro-B) cell

Precursor B

(pre-B) cell

Immature

B cell

Mature

B cell

Activated

B cell

Blast

B cell

Plasma cell

Secondary (peripheral) lymphoid organs

IgM

IgD IgM IgM

IgD

IgM

αβ

Immature T cells ± selection

Mature T cells

Effector T

(Te) cells

μ μ

β β

CD^4 CD^4

CD8

αβ Activation in secondary

lymphoid organs

(via contact and/

Thymic medullaor interleukins)

Antigen-dependent

μ

λ 5 /Vpre B 1 , 2 λ or κ

ρTα

Bone marrow

Fig.2. 2 Alllymphoidcellsoriginatefrompluripotentstemcellspresentinthe

bonemarrowwhichcanundergodifferentiationintoBorTcells.Stemcells

thatremaininthebonemarrowdevelopintomatureBcellsviaseveralanti-

gen-independentstages;includingthek5Vpre-Bcellstage,andpre-Bcells

withaspecialk 5 precursorchain.Antigencontactwithinsecondarylymphoidor-

ganscanthenactivatethesecells,finallycausingthemtodifferentiateintoanti-

body-secretingplasmacells.

Tcellsmatureinthethymus;pTaisaprecursorachainassociatedwithTCRbchain

surfaceexpression.ThepTachainislaterreplacedbythenormalTCRachain.

ImmatureCD4+CD8+double-positivethymocytesarelocalizedwithinthecortical

regionofthethymus;someautoreactiveTcellsaredeletedinthecortex,whilst

somearedeletedinthemedullaasmaturesingle-positiveTcells.Theremaining

TcellsmaturewithinthemedullatobecomeCD4+CD8–orCD4–CD8+Tcells.

Fromhere,thesesinglepositiveTcellscanemigratetoperipheralsecondary

lymphoidorgans,wheretheymaybecomeactivatedbyacombinationofantigen

contacts,secondarysignals,andcytokines.

2

Kayser, Medical Microbiology © 2005 Thieme