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106 SECTIon II Immunology ` Immunology—Immune reSponSeS Immunology ` Immunology—Immune reSponSeS

Complement System of hepatically synthesized plasma proteins that play a role in innate immunity and
inflammation. Membrane attack complex (MAC) defends against gram ⊝ bacteria. The CH 50 test
is used to screen for activation of the classical complement pathway.
actIVatIon pathWayS Classic—IgG or IgM mediated. GM makes classic cars.
Alternative—microbe surface molecules.
Lectin—mannose or other sugars on microbe
surface.
FunctIonS C3b—opsonization. C3b binds to lipopolysaccharides on bacteria.
C3a, C4a, C5a—anaphylaxis.
C5a—neutrophil chemotaxis.
C5b-9 (MAC)—cytolysis.
Opsonins—C3b and IgG are the two 1° Opsonin (Greek) = to prepare for eating.
opsonins in bacterial defense; enhance
phagocytosis. C3b also helps clear immune
complexes.
Inhibitors—decay-accelerating factor (DAF,
aka CD55) and C1 esterase inhibitor help
prevent complement activation on self cells
(eg, RBCs).

D

B Bb
C3
Alternative
C3 C3b C3bBb C3bBb3b
Spontaneous and (C3 convertase) (C5 convertase)
microbial surfaces
Amplifies generation of C3b
C3a
C5a C6-C9
Lectin
C1-like C5 C5b MAC Lysis,
complex C3b cytotoxicity
Microbial surfaces (C5b-9)
(eg, mannose) C4a
C4 C4b C3a
Classic C4b2b C4b2b3b
C1 C1 (C3 convertase) (C5 convertase)
Antigen-antibody
complexes
*
C2 C2b C3
*
C2a *Historically, the larger fragment of C2 was
called C2a but is now referred to as C2b.

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