digestion.OpsonizationinvolvesthecoatingofsuchmicrobeswithFc-ex-
pressingantibodieswhichfacilitatestheirphagocytosisbygranulocytes.
Manycells,particularlyphagocytes(andinterestinglyenoughalsosomebac-
terialikestaphylococci),bearsurfaceFcreceptorsthatinteractwithdifferent
Igclassesandsubclasses.MastcellsandbasophilsbearIgEmolecules,and
undergoaprocessofdegranulationfollowinginteractionwithallergens
againstwhichtheIgEmoleculesaredirected.Thisinducesthereleaseof
pharmacologicallyactivebiogenicamines(e.g.,histamine).Inturn,these
aminesrepresentthecausativeagentforphysiologicalandclinicalsymptoms
observedduringallergicreactions(seealsotypesI-IV,p. 1 08ff.).
TheComplementSystem...................................
Thecomplementsystem(Csystem,Fig.2. 16 )representsanon-specificde-
fensesystemagainstpathogens,butcanalsobedirectedtowardspecifictar-
getsbyantibodies.Itismadeupofaco-operativenetworkofplasmaproteins
andcellularreceptors,andislargelychargedwiththefollowingtasks:
&Opsonizationofinfectiouspathogensandotherforeignsubstances,with
theaimofmoreefficientpathogenelimination.Boundcomplementfactors
can:enhancethebindingofmicrobestophagocytozingcells;resultinthe
activationofinflammatorycells;mediatechemotaxis;inducereleaseof
inflammatorymediators;directbactericidaleffects;andinducecelllysis
(Fig.2. 17 ,p.88).
86 2 BasicPrinciplesofImmunology
Fig.2. 16 Theclassicactivationpathwayisinitiatedbyantigen-antibodycom-
plexes,thealternativepathwaybycomponentsofmicrobialpathogens.Thepro-
ductionofaC3convertase,whichsplitsC3intoC3aandC3b,iscommontoboth
pathways.C3bcombineswithC3convertasetogenerateC5convertase.C5b,pro-
ducedbyC5convertase,bindstothecomplementfactors 6 – 9 toformamem-
braneattackcomplex(MAC).C3bdegradationproductsarerecognizedbyrecep-
torsonBlymphocytes;theystimulatetheproductionofantibodiesaswellas
pathogenphagocytosis.ThecleavageproductsC3aandC4aarechemotactic
intheiraction,andstimulateexpressionofadhesionmolecules.
Nomenclature:thecomponentsofthealternativepathway(orcascade)aredesig-
natedbycapitalletters(B,D,H,I;Pforproperdin),thoseoftheclassicalpathway
(orcascade)plusterminallysisaredesignatedby“C”andanArabicnumeral(1–9).
Componentfragmentsaredesignatedbysmallletters,wherebythefirstfragment
tobesplitoff(usuallyoflowmolecularweight)istermed“a”(e.g.,C3a),there-
maining(stillbound)partiscalled“b”(e.g.,C3b),thenextsplit-offpiece“c,”and
soon.Moleculesoftengrouptoformcomplexes;intheirdesignationstheindi-
vidualcomponentsarelineduptogetherandareusuallytoppedbyaline. "
2
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Kayser, Medical Microbiology © 2005 Thieme
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