Medical Microbiology

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

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