Medical Microbiology

DefenseMechanisms 399

Antibody-DependentEnhancementofViralInfection

Thediseaseprocesscanalsobeworsenedwhenvirusesreactwithsubneutralizing

amountsortypesofantibodies.TheFcfragmentoftheantibodiesboundto

thevirusescanthenreactwiththeFcreceptorsonspecificcells.Thismakesit

possibleforcelltypestobeinfectedthatareprimarilyresistanttothevirusinques-

tionbecausetheypossessnoviralreceptors(butinanycaseFcreceptors).This

process—called“antibody-dependentenhancementofviralinfection”orADE,re-

flectingthefactthattheantibodiesexacerbatetheinfection—hasbeenexperimen-

tallyconfirmedwithanumberofvirustypestodate,includingherpesvirus,pox-

virus,reovirus,flavivirus,rhabdovirus,coronavirus,bunyavirus,andHIVspecies.

Virusexcretion.Excretionofnewlyproducedvirusesdependsontheloca-

lizationofviralreplication.Forexample,virusesthatinfecttherespiratory

tractareexcretedinexpiredair(dropletinfection).Itmustberemembered

thatingeneralizedinfectionsnotonlythetargetorganisinvolvedinexcre-

tion,butthatprimaryviralreplicationattheportalofentryalsocontributes

tovirusexcretion(forexampleenteroviruses,whichreplicateprimarilyinthe

intestinalwallandareexcretedinfeces).Onceagain,sincethesymptomsofa

viraldiseaseresultfromcelldestruction,production,andexcretionofnew

virusprogenyprecedetheonsetofillness.Asarule,patientsaretherefore

contagiousbeforetheyreallybecomeill.

DefenseMechanisms

&Themechanismsavailabletothehumanorganismfordefenseagainst

viralinfectioncanbeclassifiedintwogroups.Thenonspecificimmune

defenses,inwhichinterferonsplayaveryimportantpart,comefirst.Besides

theireffectsoncellgrowth,immuneresponse,andimmunoregulation,these

substancescanbuildupatemporaryresistancetoaviralinfection.Interfer-

onsdonotaffectvirusesdirectly,butratherinducecellularresistancemech-

anisms(synthesisof“antiviralproteins”)thatinterferewithspecificstepsin

viralreplication.Thespecificimmunedefensesincludethehumoralimmune

system,consistingmainlyofantibodies,andthecellularimmunesystem,re-

presentedmainlybytheTlymphocytes.Inmostcases,cellularimmunityis

moreimportantthanhumoralimmunity.Thecellularsystemiscapableof

recognizinganddestroyingvirus-infectedcellsonthesurfacesofwhichviral

antigensareexpressed.Thehumoralsystemcaneliminateonlyextracellular

viruses. &

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Kayser, Medical Microbiology © 2005 Thieme