Complement component 1 (C1) (C1)

The Complement component 1 (C1) is a multi-protein complex that serves as the initiator of the classical complement pathway, a vital component of the innate immune system (StatPearls, NBK554487). It consists of one C1q molecule and two molecules each of the serine proteases C1r and C1s, forming the C1q-C1r2-C1s2 complex (UniProt, P09871). Activation occurs when C1q binds to the Fc region of antigen-bound IgM or IgG, or directly to certain pathogens and apoptotic cells. This binding triggers the auto-activation of C1r, which subsequently activates C1s (PubMed, 25103611). The activated C1s enzyme then cleaves complement components C4 and C2, leading to the formation of C3 convertase and the eventual elimination of pathogens through opsonization and lysis. Dysregulation or overactivation of the C1 complex is implicated in various autoimmune and inflammatory diseases, such as cold agglutinin disease and systemic lupus erythematosus (PubMed, 31515318). Therapeutic targeting of C1, particularly through the inhibition of C1s by monoclonal antibodies like sutimlimab, has emerged as a strategy to treat complement-mediated hemolytic disorders (DrugBank, DB15581).

Other names

C1 complexC1qrsC1 esteraseComplement C1

Inhibition of the C1 complex enzymatic activity, specifically the C1s serine protease, to block the classical complement cascade.