Figure 1.

The complement activation pathways: classical, alternative, and lectin pathway. The classic pathway is triggered by binding of antigen–antibody complexes to C1q. The lectin pathway begins with signal recognition by oligomeric structures of MBL, ficolins, and collectins, which active MASP1 and MASP2, thus in turn mediate the production of C4b. Both pathways then lead to the formation of common C3 convertase, C4b2a complex. In the alternative pathway, a small fraction of the C3 molecules is hydrolyzed, exposing new binding sites and then combined with factor B protease. After cleaved by factor D, another C3convertase (C3bBb) is formed, leading to cleavage of further C3, and this process is perpetuated through an amplification loop. All three pathways ultimately result in the cleavage of C3 and C5, leading to the formation of MAC, which inserts into membrane and then induces cell lysis.