Ontogeny of the human complement system: in vitro biosynthesis of individual complement components by fetal tissues.

The human fetal liver is capable of synthesizing the biologically active form of the second (C2) and fourth (C4) components of complement as early as 8 wk after conception, and the inhibitor of C1 (C1 INH) as early as 11 wk after conception. Biologically active C3 was produced in vitro by fetal liver obtained at 14 wk gestation. These conclusions were based on the observations that isolated fetal livers produced biologically active C2, C3, C4, and C1 INH, that this production was temperature dependent and reversibly inhibited by well-known inhibitors of protein synthesis, and that (14)C-labeled amino acids were incorporated into proteins immunochemically identical with these proteins. The data suggested that a large mononuclear cell was the cell type in the fetal liver that synthesized C2 and C4.