Medical Microbiology

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T-CellMemory..............................................


AswithBcellsandantibodies,enhanceddefensesagainstintracellularpatho-
gens(especiallyvirusesandintracellularbacteria)doesnotsolelydependon
increasednumbersofspecificTcells,butratherisdeterminedbytheactiva-
tionstatusofTcells.Hereagainitmustbeemphasizedthatprotectiveim-
munologicalmemoryagainstmostbacteria,bacterialtoxins,andviruses,
ismediatedbyantibodies!MemoryTcellsarenonethelessimportantin
thecontrolofintracellularbacterialinfections(e.g.,tuberculosis[TB],leprosy),
aswellaspersistentnoncytopathicvirusessuchashepatitisBandHIV(see
alsop. 10 6).Ithasbeendemonstrated,atleastinmousemodels,thatahigher
numberofTcellsaloneisofteninsufficientfortheprotectionofthehost
againsttheimmunopathologicalconsequencesofadefensiveCD8+T-cellre-
sponse.YetsuchTcellresponsesmustbeactivatedinordertoprovideim-
munity.Inthecaseoftuberculosis,sustainedactivationofacontrolledT-cell
responsebyminimalinfectionfociwaspostulated,andconfirmed,inthe
1 960sasconstitutinginfectionimmunity—i.e.thelifelong,andusuallyef-
fective,immunecontrolofthediseasebyanongoinglocalizedlow-levelof
infection.Asimilarsituationisobservedforcell-mediatedimmuneresponses
againstleprosy,salmonellae,andnumerousparasiticdiseases(oftentogether
withantibodies).Theexistenceofinfection-immunityexplainswhyappar-
entlycontrolled,minimal,infectionstendtoflareupwhentheimmunesys-
temiscompromisedbycytostaticdrugs,age,orHIVinfection.Delayedtype

98 2 BasicPrinciplesofImmunology

IgSerumConcentrationCurve

An

tib

od

y^ c

on

ce

nt

ra

tio

n^

as

ap

er

ce

nt

ag

e^ o

f

adult le

vels

–3
Time (months)

0 6 12 18

0

50

100

IgA, IgD, IgE

IgG

IgM

Fig.2. 18 Synthesisofsig-
nificantamountsofimmu-
noglobulins only begins
duringtheperinatalperiod
(uninterruptedlines). IgG
fromthemotheristhere-
forethechild’smainmeans
ofprotectiveimmunitybe-
foretheageofthreetosix
months(dottedline).Infec-
tionsencounteredduring
thisearlyperiodareattenu-
atedbymaternalantibo-
dies,renderingsuchinfec-
tionsvaccine-like.

2


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Kayser, Medical Microbiology © 2005 Thieme
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