Fig. 1.

The activation of complement pathway. There are 3 different pathways for complement activation: the classical pathway, the alternative pathway, and the lectin pathway. The triggers of activation are: as follows: (1) the antigen/antibody complex binding to the C1q of the C1 complex in the classical pathway; (2) foreign carbohydrate moieties binding to mannose binding lectin or ficolin in the lectin pathway; and (3) spontaneous hydrolysis of C3 in the alternative pathway. These 3 pathways cause the cleavage of C3 into C3a and C3b. C3b binds to C5 convertase C4b1a3b or C3bBb3b, leading to the cleavage of C5 into C5a and C5b. C5b initiates the lytic pathway and form C5b-9 the membrane attack complex (MAC), leading to direct lysis. Eculizumab and ravulizumab are complement-targeted monoclonal antibodies for inhibiting the cleavage of C5 into C5a and C5b.