Fig. 1.

The three pathways of the complement cascade. The complement system has three main complement activation pathways: the classical pathway (CP), the lectin pathway (LP), and the alternative pathway (AP). The CP is triggered by activation of the C1-complex (C1q–C1r–C1s). The LP is homologous to the CP and activated by mannose-binding lectin (MBL), ficolins and mannose-binding protein-associated serine proteases (MASPs). The AP is continuously activated by low-level autoactivation of C3. All the three pathways merge into a common terminal pathway with the activation of C5 into C5a and C5b. C5b binds to C6 and C7 and interacts with C8 and C9 to form the membrane attack complex (MAC)