Medical Microbiology

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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