Complement C4c is a proteolytic fragment derived from complement component C4. As a central mediator of classical and lectin complement pathways, C4 is cleaved during activation into anaphylatoxin C4a and opsonin C4b. Subsequent processing of C4b by Factor I yields stable C4c (146 kDa) and C4d, with C4c serving as a soluble biomarker of systemic complement activation.

C4 activation bridges innate and adaptive immunity: C4b facilitates C3 convertase formation for pathogen opsonization, while C4a induces inflammatory responses. The C4A and C4B isotypes exhibit distinct binding specificities-C4A targets amino groups on immune complexes, whereas C4B binds hydroxyl groups on microbial surfaces.