The complement system. Complement regulators are indicated in red. MBL: mannan-binding lectin; MASP: MBL-associated serine protease; C4BP: C4b-binding protein; CR1: complement receptor 1. The complement system consists of 3 initiating pathways: classical pathway, lectin pathway, and alternative pathway. The classical pathway is usually activated by antigen-antibody complexes, the lectin pathway is activated by microbes with MBL-MASP complex, and the alternative pathway is activated spontaneously by hydrolysis of C3 to C3(H2O). All 3 pathways lead to formation of C3 convertase, followed by C5 convertase, ultimately leading to formation of membrane attack complex. In this process, anaphylatoxins C3a and C5a are also released. The complement system is kept in check by a number of regulators.
