The enzyme C1 is
able to cleave C4 into C4a and C4b. The C4b binds to adjacent proteins and
carbohydrates on the surface of the antigen. C2 then binds to the C4b and
C1 cleaves C2 into C2a and C2b. The C4b2a functions as a C3 convertase that
can subsequently cleave hundreds of molecules of C3 into C3a and C3b.
