C3b
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. The cleavage of C3 leaves C3b with an exposed thioester bond, allowing C3b to effectively coat and tag foreign cells by covalently binding to -OH and -NH2 groups on foreign cell surfaces. 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 also leads to the recruitment of complements C5, C6, C7, C8, and C9, all of which form a complex that results in a pore through the antigen's membrane. This pore disrupts the ionic and osmotic balance provided by the antigen's membrane, and leads to the death of the antigen cell
C3b is cleaved into C3c and C3d.
