Molecular model showing human complement C3b (pale colour) bound to an infected cell membrane in complex with Smallpox Inhibitor of Complement (SPICE, orange). C3b's complement protein ability to perform its functions derives from its ability to covalently bind to the surface of invading pathogens within an organism's body. The cleavage of C3 leaves C3b allow C3b to effectively coat and tag foreign cells by covalently binding to hydroxyl (-OH) and amine (-NH2) groups on foreign cell surfaces. Regulators of complement activation (RCA) like SPICE inhibit complement-induced immune responses on healthy host tissues.