Medical Microbiology

andtumorsismorelikelytobemediatedviaperforinsandcytolysis.How-

ever,cytotoxicimmuneresponsescanalsocausedisease,especiallyduring

noncytopathicinfections.Developmentofanevolutionarybalancebetween

infectiousagentsandimmuneresponsesisanongoingprocess,asreflected

bythenumerousmechanismsemployedbypathogensandtumorstoevade

immune-mediateddefenses. &

Allimmunedefensemechanisms(seeFig.2. 1 ,p. 4 4)areimportantinthebat-

tleagainstinfections.Naturalhumoralmechanisms(antibodies,comple-

ment,andcytokines)andcellularmechanisms(phagocytes,naturalkiller

cells,Tcells)aredeployedbytheimmunesystemindifferentrelative

amounts,duringdifferentphasesofinfection,andinvaryingcombinations.

Grosssimplificationsarenotveryhelpfulintheimmunologicalfield,but

asmallnumberoftenablerulescanbedefinedbasedoncertainmodelin-

fections.Suchmodelsaremainlybasedonexperimentscarriedoutinmice,

oronclinicalexperiencewithimmunodeficientpatients(Fig.2. 19 ).

GeneralRulesApplyingtoInfectionDefenses............

&Non-specific defenses arevery important(e.g., Toll-like receptors,

IFNa/b),and’naturalimmunity’(meaningnotintentionallyorspecifically

induced)representedbynaturalantibodies,directcomplementactivation,

NKcellandphagocytes,playsasignificantroleinallinfections.However,

muchremainstobelearnedabouttheirroles.

&Antibodiesrepresentpotenteffectormoleculesagainstacutebacterialin-

fections,bacterialtoxins,viralre-infections,andinmanycasesagainstacute

cytopathicprimaryviralinfections(e.g.,rabiesandinfluenza).Antibodiesare

alsolikelytomakeamajorcontributiontothehost-parasitebalanceoccur-

ringduringchronicparasiticinfections.IgAisthemostimportantdefense

mechanismatmucosalsurfaces(Fig.2. 5 ,p. 57 ).

&Perforin-dependentcytotoxicityinCD8+Tcellsisimportantfordefense

againstnoncytopathicviruses,forthereleaseofchronicintracellularbacteria,

andforprotectionagainstintracellularstagesofcertainparasites.

&NonlyticT-cellresponsesprovideprotectionintheformofcytokines(very

importantcytokinesincludeIFNcandTNFa),whichpromotetheenhanced

digestionanddestructionofintracellularbacteriaandparasites(e.g.,listeria,

leishmania,etc.),andinsomesituationsenhanceimmunityagainstcomplex

viruses(e.g.,thesmallpoxvirus)(Fig.2. 15 ,p.79).Infectiousagentsapparently

inducecytokineswithinamatterofhours(forinstanceIFNc,IL-1 2 ,andIL-4),

andthisearlycytokineproductioninturnfunctionstodefinetheensuing

Tcellresponseastype 1 ortype 2 (seep. 75 andFig.2. 14 ,p.78).

100 2 BasicPrinciplesofImmunology

2

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

Kayser, Medical Microbiology © 2005 Thieme

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