C3 and complement component 3 deficiency: Increased susceptibility to both Gram-positive and Gram-negative bacterial infection have been described with C3 deficiency.27,28 Osteoblasts can be stimulated to produce C3 by 1,25-dihydroxyvitamin D.29 In culture experiments when anti-C3 antibody was used, osteoclast formation was greatly inhibited.30There has been no evidence for cleavage of C3 in either osteoblasts or osteoclasts.31 C1-s can cleave type I and type II collagen32 but the possible role of any component of the complement system in the mineralization of collagen is unknown.