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).
7
Kayser, Medical Microbiology © 2005 Thieme