Fig. (1). Introduction to the complement system.

In the classical pathway, C1q recognizes and binds to immunoglobulins (IgG and IgM) presented on damaged or invading cells. The C1q-antibody complex activates serine proteases C1s and C1r which then combine C2 and C4 into the classical C3 convertase (C4b2a). In the lectin pathway, MBL recognizes and binds to glycans presented as antigens on damaged cells or microorganisms. The MBL-glycan complex activates serine protease MASP2 to create the lectin C3 convertase from C2 and C4 (C4b2a). The alternative pathway greatly differs from classical and lectin pathways. With increased concentrations of circulating C3(H₂O), factors B and D spontaneously form the alternative C3 convertase (C3bBb). Anaphylatoxins C3a, C4a, and C5a are created through enzymatic cleavage of C3, C4, and C5; while C3b, C4b, and C5b are necessary for continued propagation of the CS toward the terminal pathway; C3a, C4a, and C5a carry out secondary CS actions to activate other immune systems. The sequential cascade of CS is highlighted by the solid arrows while the dotted arrows identify the origins of anaphylatoxin production (Fujita, 2004; Sunyer et al, 2005). The terminal pathway begins with C5 cleavage by C5 convertase and ends with MAC formation resulting in targeted cytolysis.