Complement fragment C3d (C3d)

Complement fragment C3d is a cleavage product of complement component C3, generated during activation of the complement cascade. C3d contains a thioester domain that allows it to covalently bind to cell surfaces, especially microbial surfaces. In humans, C3d functions as an opsonin, marking antigens for recognition by immune cells, and plays a pivotal role in bridging innate and adaptive immunity. By binding to the B cell coreceptor CD21 (CR2), C3d increases the sensitivity of B cells to antigens, enhancing antibody production up to 1,000–10,000 fold. Its interaction with complement regulatory proteins and bacterial proteins (such as Staphylococcus aureus Efb-C) illustrates both its protective and regulatory roles, as well as a target for pathogen evasion mechanisms. C3d levels and binding patterns are used as indicators of complement activation and immune status in disease.

Other names

Complement C3dC3dg (when referring to the cleavage variant that includes C3d domain)complement fragment C3d

Drugs targeting complement (e.g., recombinant inhibitors, monoclonal antibodies) may block C3d formation or interrupt C3d interaction with CR2/CD21, thereby modulating B cell activation and immune response.