The activation of C1 gives C1s enzymatic activity to cleave complement protein
C4 into C4a and C4b. The C4b binds to adjacent proteins and carbohydrates on
the surface of the antigen. C4b can attach antigens to phagocytes for
opsonization (enhanced attachment). C4a can weakly promote inflammation.
C1q
activated C1r
activated C1s
C4