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 Ca(2+)-dependent C1r(2)C1s(2) proenzyme complex, and efficient activation of C1 takes place on interaction of the globular heads of C1q with the Fc regions of IgG or IgM antibody present in immune complexes. C1 is a calcium-dependent trimolecular complex of C1q, C1r and C1s in the molar ration of 1:2:2. C1q subcomponent is composed of nine subunits, six of which are disulfide-linked dimers of the a and B chains, and three of which are disulfide-linked dimers of the C chain. In addition to the major A:B and C:C dimer bands, rat, unlike human C1q, contained minor dimer species. Highest levels in spleen, lung and brain. Weaker expression in kidney and liver. In the spleen, localized mainly to the red pulp, in cells mainly of monocyte-macrophage lineage. In white pulp, localized in specific dendritic cells such as those from the periarteriolar lymphatic sheath (PALS). Hydroxylated on lysine and proline residues. Hydroxylated lysine residues can be glycosylated.