C4b is a protein fragment that plays a crucial role in the immune system, specifically in the complement system. It is generated from the cleavage of the protein C4 during the activation of the classical and lectin pathways of complement activation. C4b binds to the surface of pathogens, marking them for destruction by immune cells. Once attached to a pathogen, C4b can interact with other complement proteins, leading to the formation of the C3 convertase enzyme. This enzyme further activates the complement cascade, enhancing the immune response by promoting inflammation and facilitating the clearance of pathogens from the body.