Medical Microbiology

402 7 GeneralVirology

responseandvirusesthatdonotchangetheantigenicityoftheirhostcells

tendtoactivatethehumoralsystem.

Humoralimmunity.Antibodiescanonlyattackvirusesoutsideoftheirhost

cells,whichmeansthatonceaninfectionisestablishedwithinanorganitcan

hardlybefurtherinfluencedbyantibodies,sincethevirusesspreaddirectly

fromcelltocell.Inprinciple,thehumoralimmunesystemisthusonlycapable

ofpreventingageneralizedinfection,butonlyiftheantibodiesarepresentat

anearlystage(e.g.,inducedbyavaccination).ClassIgGandIgMantibodies

areactiveinthebloodstream(seeChapter2)andclassIgAisactiveonthe

mucosalsurface.Theeffectoftheantibodiesontheviralparticles(“neutra-

lization”)isbasedonsterichindranceofvirusadsorptiontothehostcellsby

theantibodiesattachedtotheirsurfaces.Theneutralizingeffectofantibodies

isstrongestwhentheyreactwiththereceptor-bindingsitesonthecapsidsso

astoblockthem,renderingthevirusincapableofcombiningwiththecellular

receptors(p.384).

Cellularimmunity.Thistypeofimmunedefenseisfarmoreimportantwhen

itcomestofightingviralinfections.Tlymphocytes(killercells)recognize

virus-infectedcellsbytheviralantigensontheirsurfacesanddestroy

them.Theobservationthatpatientswithdefectivehumoralimmunitygen-

erallyfarebetterwithvirusinfectionsthanthosewithadefectivecellular

responseunderlinesthefactthatthecellularimmunedefensesystemis

themoreimportantofthetwo.

Prevention

&Themostimportantprophylacticmeasuresinthefaceofpotentialviral

infectionsareactivevaccines.Vaccinescontaininginactivatedvirusesgener-

allyprovideshorter-livedandweakerprotectionthanlivevaccines.Passive

immunizationwithhumanimmunoglobulinisonlyusedinasmallnumber

ofcases,usuallyaspostexposureprophylaxis. &

Valueofthedifferentmethods.Ingeneral,vaccination,i.e.,inductionofim-

munity(immuneprophylaxis)isthemostimportantfactorinpreventionof

viralinfection.Exposureprophylaxisisonlyrelevanttohygienicmeasuresne-

cessitatedbyanepidemicandisdesignedtopreventthespreadofpathogens

inspecificsituations.Chemoprophylaxis,i.e.,administrationofchemothera-

peuticagentswhenaninfectionisexpectedinsteadofafterithasbeendi-

agnosedtoblockviralmetabolism,isnowjustifiedinselectedcases,e.g.,in

immunosuppressedpatients(seeChemotherapy,p. 40 4).

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Kayser, Medical Microbiology © 2005 Thieme