Medical Microbiology

SomecomponentsmediategeneralregulatoryfunctionsonB-cellresponses,

especiallyviaCR1andCR2.

ImmunologicalCellDeath..................................

Fig.2. 17 summarizesthemechanismsofcelldeathresultingfromimmuno-

logicalcellinteractionsanddifferentiationprocesses,astheyareunderstood

todate.

ImmunologicalTolerance...........................................

&T-celltolerance,asdefinedbyalackofimmunereactivitycanbedueto

anumberofprocesses:Firstly,Negativeselectioninthethymus(referredto

asdeletion);secondlyasimplelackofreactivitytoantigen(selfornonself)as

aresultoftheantigenhavingnotbeenpresentinthesecondarylymphoid

organsinasufficientquantityorforasufficientamountoftime;andthirdly

anexcessivestimulationofT-cellsresultingfromtheubiquitouspresence

ofsufficientantigenresultinginTcellexhaustion.Finally,itmayalsobe

possiblethatTcellscanbecometemporarily“anergized”bypartialorincom-

pleteantigenstimulation.Asageneralrule,self-reactive(autoimmune)

Bcellsarenotgenerallydeletedbynegativeselectionandcantherefore

bepresentintheperiphery.ExceptionstothisruleincludeBcellsspecific

formembrane-boundself-determinants,someofwhicharedeletedoraner-

gized.Bcellsreactpromptlytoantigens,evenself-antigens,whicharear-

rangedrepetitively.However,theyonlyreacttosolublemonomericantigens

iftheyadditionallyreceiveTcellhelp.Thus,B-cellnon-reactivitylargely

resultsfromalackofpatternedantigenpresentationstructuresorasaresult

ofT-celltolerance. &

Immunologicaltolerancedescribestheconceptthattheimmunesystemdoes

notnormallyreacttoautologousstructures,butmaintainstheabilitytoreact

againstforeignantigens.Toleranceisacquired,andcanbemeasuredasthe

selectiveabsenceofimmunologicalreactivityagainstspecifiedantigens.

T-CellTolerance............................................

Adistinctioncanbemadebetweencentraltolerance,whichdevelopsinthe

thymusandisbasedonthenegativeselection(deletion)ofTcellsrecognizing

selfantigenspresentinthethymus,andperipheraltolerance.Peripheral

toleranceresultsinthesameoutcomeascentraltolerance,however,this

90 2 BasicPrinciplesofImmunology

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Kayser, Medical Microbiology © 2005 Thieme

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