Figure 2.

Schematic representation of the classical complement cascade, showing the initiation of the pathway by binding of the C1q component of C1 binding to the Fc region of an immunoglobulin G (IgG) antibody molecule bound to an antigen on the surface of a target cell. The numbers in bold indicate the sequential steps that are involved in the activation of each of the components, leading to the final lytic event carried out by the membrane attack complex (MAC). Some sources now use a revised nomenclature for the fragment of C2, in which C2a is the small fragment that diffuse away, and C2b is the larger fragment that binds with C4b and acts in the convertase, C4b2b (Source: Refs. 16 and 17).