C3b

C3b is a one of the elements formed by the cleavage of complement component 3.

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.