Composante complémentaire 1

Complement component 1 (C1) is the first component of the classical complement pathway, assembled as a calcium-dependent complex of one C1q molecule and two copies each of the serine protease proenzymes C1r and C1s. C1q, a flower-bunch-shaped protein with six collagen-like stalks and globular heads, recognizes immune complexes, apoptotic debris, and certain pathogens, triggering autoactivation of C1r, which then cleaves C1s. Activated C1s subsequently cleaves C4 and C2 to form the C3 convertase C4b2a. Deficiency of C1q is strongly associated with systemic lupus erythematosus and severe recurrent infection, and recombinant or purified C1q subcomponents are used in complement pathway assays and autoantibody detection.