Figure 1.

The classical complement pathway

C1q of the C1 complex binds and coats cell membranes, activating Cr and Cs serine proteases. This initiates cleavage of C2 and C4 into peptides (C2b and C4b) that recombine to form the C3 convertase complex. C3 convertase catalyses C3 into C3a and C3b, the latter of which tags synapses for phagocytic elimination through interaction with CR3. A more extensive overview can be found in Nature Reviews Immunology.⁴