The classical and alternative complement pathways.
C3b is the larger of two elements formed by the cleavage of complement component 3. C3b covalently bonds to microbial cell surfaces within an organism's body. This leads to the production of surface-bound C3-convertase and thus more C3b components. Bound C3b also aids in opsonization of the microbe by macrophages. Complement receptor 1 (CR1) on macrophages allows the engaging of C3b covered microbes. C3b also aids in the clearance of immune complexes (antigen-antibody).
C3b is cleaved into C3c and C3d.