Medical Microbiology

thecytomegalyvirus,encodeproteinsthatarefunctionallyanalogousto
chemokinereceptors.Thisallowsarapidneutralizationoflocallyinduced
chemokines,andmayofferanadvantagetothevirus.TheDuffyantigenre-
ceptorforchemokines,DARC,isexpressedonendothelialcellsandiscapable
ofahigh-affinitybindinginteractionwithvariouschemokinetypes.Since
thisreceptorhasnodownstreamsignalingcascade,itisassumedtofunction
inthepresentationofchemokinestoleukocytesastheyflowpast.DARCalso
functionsasareceptorforPlasmodiumvivax.CCR5andCXCR4areco-recep-
torsforHIVinfectionofCD4+Tcells.

Antibody-DependentCellularImmunity

andNaturalKillerCells

LymphocytescannonspecificallybindIgGantibodiesbymeansofFcrecep-
tors,thenspecificallyattacktargetscells(e.g.,infectedortransformedcells)
usingtheboundantibody.Thisphenomenon,knownasantibody-depen-
dentcellularcytotoxicity(ADCC),hasbeendemonstratedinvitro—however
itsin-vivofunctionremainsunclear.Naturalkiller(NK)cellsalsoplayarole
inADCC.ThegenesisofNKcellsappearstobemainlythymus-independent.
ThesecellscanproduceIFNcveryearlyfollowingactivationanddonot
requireaspecificreceptor.Thesecellsarethereforeearlycontributorsto
theIFNc-orientedTH1immuneresponse.NKcellscanrespondtocells
thatdonotexpressMHCclassImolecules,andareinactivatedbycontact
withMHCmolecules.Thisrecognitionprocessfunctionsviaspecialreceptors
thatarenotexpressedinaclonalmanner.NKcellsprobablyplayanimpor-
tantroleintheearlydefensivestagesofinfectiousdiseases,althoughthe
exactnatureoftheirroleremainstobeclarified(virus-inducedIFNaand
IFNbpromoteNKactivation).NKcellsalsoappeartocontributetorejection
reactions,particularlytherejectionofstemcells.

Humoral,Antibody-DependentEffector

Mechanisms

Theobjectivesoftheimmuneresponseinclude:theinactivation(neutraliza-
tion)andremovalofforeignsubstances,microorganisms,andviruses;the
rejectionofexogenouscells;andthepreventionofproliferationofpatho-
logicallyalteredcells(tumors).Thesystemsandmechanismsinvolvedin
theseeffectorfunctionsarelargelynon-specific.Specificimmunerecognition
byBandTcellsdirectstheseeffectormechanismstospecifictargets.Forin-
stance,immunoglobulinsopsonizemicrobes(e.g.,pneumococci)whichare
equippedwithpolysaccharidecapsulesenablingthemtoresistphagocyte

ImmuneResponsesandEffectorMechanisms 85

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