Binding of antibodies to multivalent antigen; binding of C1 to antibodies
Binding of C4 to Ig-associated C1q
Cleavage of C4 by C1r2S2 enzyme; covalent attachment of C4b to antigenic surface and to antibodies
Binding of C2 to C4; cleavage of C2 to form C4b2b complex (C3 convertase)
Cleavage of C3 by C3 convertase
Binding of C3b to antigenic surface and to C4b2b complex i_i
Cleavage of C5; initiation of late steps of complement activation
convertase cir^^rn n
FIGURE 12-9 The classical pathway of complement activation. Antigen-antibody complexes that activate the classical pathway may be soluble, fixed on the surface of cells (as shown), or deposited on extracellular matrices. The classical pathway is initiated by the binding of C1 to antigen-complexed antibody molecules, which leads to the production of C3 and C5 convertases attached to the surfaces where the antibody was deposited. The C5 convertase cleaves C5 to begin the late steps of complement activation.
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