C1q associates with the proenzymes C1r and C1s to yield C1, the first component of the serum complement system. The collagen-like regions of C1q interact with the Ca2+-dependent C1r2C1s2 proenzyme complex, and efficient activation of C1 takes place on the interaction of the globular heads of C1q with the Fc regions of IgG or IgM antibody present in immune complexes.
