Medical Microbiology

&Solubilizationofotherwiseinsolubleantigen-antibodycomplexes.

&Promotionofthetransportofimmunecomplexes,andtheirelimination
anddegradation.

&Regulationoftheimmuneresponse,achievedviatheirinfluenceon
antigenpresentationandlymphocytefunction.
Over 20 proteinsofthecomplementsystemhavebeenidentifiedtodate,
andareclassifiedaseitheractivationorcontrolproteins.Thesesubstances
accountforabout 5 %ofthetotalplasmaproteins(i.e., 3 – 4 g/l).C3isnot
onlypresentinthelargestamount,butalsorepresentsacentralstructure
forcomplement activation.Acleardifference existsbetween“classic”

ImmuneResponsesandEffectorMechanisms 87

TheComplementSystem:ClassicandAlternativeActivation

Classic pathway Alternative pathway

Immune

complexes

(IgG, IgM) + C1

C4 C2

C4a C4b C2a C2b

IC3b, C3c, C3d

C3dg, C3g

Degradation

C4b2a3b or C3bBb3b

C5 convertase

C5

C5a

C6–C9 C5b

Lytic complex

(MAC)

C3a

Microorganisms +

P + D (and

spontaneous)

C3 B

C3a C3b Bb

C4b2a

(classic C3

convertase)

C3bBb

(alternative

enhancing C3

convertase)

C3b

C3

Ba

En

ha

nc

em

en

t

2

All rights reserved. Usage subject to terms and conditions of license

Kayser, Medical Microbiology © 2005 Thieme

All rights reserved. Usage subject to terms and conditions of license.