Final answer:
C3d and iC3b are breakdown products of C3b, which bind to CR2 on the B-cell co-receptor, playing an important role in the immune system's response to pathogens.
Step-by-step explanation:
C3d and iC3b are breakdown products of C3b, which binds to CR2 of the B-cell co-receptor. This process is crucial for the immune response, particularly the complement system which has vital roles including the opsonization of pathogens, triggering of an inflammatory response, and formation of a membrane-attack complex (MAC). The B-cell co-receptor complex plays a key role in enhancing the cellular response to antigens, involving the molecule CR2 which specifically binds to C3d and iC3b.