Medical Microbiology

&Avoidancestrategies.Infectiousagentshavedevelopedavarietyofstra-

tegiesbywhichtheycansometimessucceedincircumventingorescaping

immuneresponses,oftenbyinhibitingcytokineaction.

AntibacterialImmuneEffectorMechanisms

Extracellularbacteria.Capsuleswithcarbohydrateelementsrenderbacteriamore

resistanttoefficientphagocytosisanddigestion(mainlybygranulocytes)—how-

ever,highlyrepetitivecarbohydratesurfaceantigensinduceefficientBcellsre-

sponseswhichdonotrequireThelpandwhicharesupportedinpartbylipopoly-

saccharides(LPS).PurecarbohydratesdonotinduceThelp!Short-livedIgMresponses

cancontrolbacteriainthebloodeffectively,butareusuallyinsufficientinthe

controloftoxins.Insuchcases,immunoglobulinsoftheIgGclassaremoreefficient,

asaresultoftheirlongerhalf-lifeandgreaterfacilityfordiffusingintotissues.

IntracellularbacteriaarecontrolledbyTcells(mainlyviaTcellsecretedIFNcand

TNFawhichactivatemacrophages),orinsomecasesbythereleaseofintracellular

bacteriathroughCD8+Tcellmediatedcellulardestruction.

AvoidanceMechanismsofPathogens(withexamples)

Influenceonthecomplementsystem.Somepathogenspreventcomplementfac-

torsfrombindingtotheirsurfaces:

&PreventionofC4bbinding;herpesvirus,smallpoxvirus.

&PreventionofC3bbinding;herpessimplexvirus(imitatesDAF,seep.86),try-

panosomes.

Compartmentalizationinnon-lymphoidorgans.Virusescanavoidconfrontation

withtheimmunedefensesbyrestrictingtheirlocationtoperipheralcellsandor-

ganslocatedoutsideoflymphoidtissues:

&Papillomaviruses;infectkeratinocytes.

&Rabiesvirus;infectsneurons.

Modulationanddown-regulationofsurfaceantigens.Infectionagentscan

avoidimmunedefensesbymutatingorreducingtheirexpressionofT-orB-cell

epitopes.

&Influenzaviruses;antigenicshiftcausedbyrearrangementofgeneticelements

ordriftresultingfrommutationofhemagglutinin(atthepopulationlevel).

&Gonococci;recombinationofpiligenes.

&Schistosoma;mutationofenvelopeproteinsormaskingbyadoptionofhost

MHCantigens.

Interferencewithphagocytosisanddigestion.Mycobacteriumtuberculosisuses

CR1,CR2,orfibronectinasareceptorforcellentry;itdoesnotinduceefficient

oxidativemechanismsinmacrophages.

&Componentsofbacterialcellwallscanimpedephagosome-lysosomefusionand

areresistanttodigestion.

&Heatshockproteins(hsp60andhsp70)orsuperoxidedismutaseaidresistance.

102 2 BasicPrinciplesofImmunology

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Kayser, Medical Microbiology © 2005 Thieme

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