16th February 2024

Summary

C3 is the essential complement pathway component; all three activation pathways form the C3 convertase, which converts C3 into C3a and C3b. C3a has anaphylatoxin activity, directly triggering degranulation of mast cells as well as increasing vascular permeability and promoting inflammation. On the other hand, C3b opsonises targets for phagocytosis. Moreover, C3b also acts as a protease by cleaving C5 into C5a and C5b. C5a functions in a similar way to C3a. C5b initiates a cascade of reactions leading to the formation of what is known as the membrane attack complex (MAC) C5b, C6, C7, C8, and C9.

Image Source: Wikimedia Commons (2006). Figure 1.4 Complement activation, Licensed under a creative commons licence. Available for free at: https://upload.wikimedia.org/wikipedia/commons/f/f5/Figure_1.4._Complement_activation.jpg (Accessed: 9 January 2024).