Figure 1

The classic and alternative pathways of the complement system. The squiggly line arrows indicate that proteolytic cleavage of the molecule at the tip of the arrow has occurred; a line over a complex indicates that it is enzymatically active. Note that the nomenclature of the cleavage products of C2 is undecided. Some call the large fragment C2a and others call it C2b. Here, all small fragments are labeled 'a' and all large fragments are labeled 'b'. Hence, the C3 convertase is depicted as C4b,2b. Note that proteases associated with the mannose-binding lectin cleave C4 as well as C2. Reprinted with permission from McGraw-Hill Medical [67].