Fig. 3.

Amphioxus complement systems are activated and amplified by the formation of C3 convertases through the C1q-mediated, lectin and alternative pathways. C3 can be cleaved by C3 convertases to form C3a and C3b. The C3a fragment has bactericidal activity and is capable of enhancing phagocyte phagocytosis against bacteria. The C3b fragment can bind covalently to cell surface carbohydrates via its reactive thioester. Amphioxus has five C6-like proteins, but it is unclear whether they participate in the formation of the membrane attack complex (MAC). A solid arrow indicates that the pathway was supported by experimental data; a dashed arrow indicates that no experimental support is present