Human complement system protein (None standardized for the entire system; individual proteins include abbreviations such as C1, C2, C3, C4, C5, C6, C7, C8, C9, Factor B (fB), Factor D (fD), Properdin (fP))

The human complement system comprises a network of approximately 50 plasma proteins and membrane receptors primarily synthesized by the liver and circulating in blood and tissues. This system, a key component of humoral innate immunity, operates through three major activation pathways—classical, alternative, and lectin—triggered by pathogen recognition or immune complexes. Activation results in a sequential proteolytic cascade amplifying the immune response and producing effector molecules: opsonins promoting phagocytosis (C3b), chemotactic and inflammatory peptides (C3a, C5a; anaphylatoxins), and the membrane attack complex (C5b-9), which mediates lysis of target cells. Complement proteins also facilitate the clearance of immune complexes and apoptotic cells, bridge innate and adaptive immunity, and play a role in tissue homeostasis. Dysregulation or genetic deficiency of complement proteins is implicated in a spectrum of human diseases, including autoimmune, infectious, and inflammatory conditions, making several system components important therapeutic targets.

Other names

Complement cascade proteinsComplement componentsSerum complement proteinsHumoral complement proteinsIndividual proteins (e.g., "C3", "C5") are commonly used as aliases for their respective subunits

Inhibition of complement activation (blocking cleavage of C5, C3, etc.). Prevention of MAC assembly and cell lysis. Reduction of inflammation (inhibiting anaphylatoxin effects).