C3 (complement)

In immunology, soluble C3-convertase, also known as iC3Bb, catalyzes the proteolytic cleavage of C3 into C3a and C3b as part of the alternative complement system.

C3a
C3a plays an important role in chemotaxis, though not as important a role as C5a.

It is also an anaphylatoxin.

C3b
C3b may bind to microbial cell surfaces within an organism's body. This can lead to the production of surface-bound C3 convertase and thus more C3b components. Also known as C3bBb, this convertase is similar to soluble C3-convertase except that it is membrane bound.

Alternatively, bound C3b may aid in opsonization of the microbe by macrophages. Complement receptor 1 or CR1 on macrophages allows the engaging of C3b covered microbes.

C3b is cleaved into C3c and C3d.