Complement Activation Pathways

There are three different complement pathways, the classical complement pathway, the alternative complement pathway, and the mannose-binding lectin pathway. The classic complement pathway is triggered when antibody-antigen complex interact with C1-complex, which consists of C1q, two molecules of C1r, and two molecules of C1s. The C1-complex cleaves C2 and C4, which then form C3 convertase (C4b2a). C3 is then cleaved by the C3 convertase, and forms C5 convertase in association with C4b and C2a. The generation of C5 convertase is the end of the classical pathway. The lectin pathway is very similar to the classical pathway. It is stimulated when the mannose-binding lectin (MBL) binds to mannose residues on the pathogen surface. The MBL-associated serine proteases, MASP-1, and MASP-2, are activated and cleave C4 and C2, which then form the C3 convertase as in the classical pathway. The alternative complement pathway begins with the activation of C3 and requires factor B and factor D. All three pathways merge at C3, which is then converted into C3a and C3b.