Editing Classical complement pathway (section)

=== C3b function and structure. ===
C3b can act as an opsonin. C3b is very similar to C4 in both structure and function also has a thioester bond that forces it to attach to surface nucleophile of the activator(namely the pathogen or IC). Phagocytes have receptors for C3b and as a result of receptor-ligand binding are able to more easily recognize and engulf pathogen molecules.    While the [[anaphylatoxin]] C3a interacts with its [[C3a receptor]] (C3aR) to recruit leukocytes, C3b contributes to further downstream complement activation.<ref name="Overview of Complement" /><ref name="Complement history" />