C3dg

C3dg is a fragment of the complement protein C3 that is produced during the proteolytic processing of C3b, one of the central opsonins of the complement system. After C3 is activated, C3b covalently binds to microbial surfaces or immune complexes. When C3b is cleaved by the serine protease factor I in the presence of cofactors such as factor H or C4b-binding protein, a series of degradation products are generated, among which C3dg remains associated with the surface as part of the C3d family of fragments. C3dg, together with related fragments like C3d, serves as a molecular tag reflecting complement activation on a surface.

The best-recognized function of C3dg is its interaction with the CD21 (CR2) receptor on B cells and follicular dendritic cells. Binding of C3dg-containing complexes to CR2 lowers the threshold for B cell activation, aiding antigen-driven antibody responses and contributing to affinity maturation and memory formation. This mechanism provides a link between innate tagging by complement and adaptive immunity.

In research and clinical contexts, C3dg and related fragments are used as markers of complement activation and opsonization. They are also explored in vaccine adjuvant strategies that seek to harness C3d/C3dg-mediated B cell co-stimulation. See also: C3d, C3, complement system, CR2.