Maturation of the human complement system. I. Onset time and sites of fetal C1q, C4, C3, and C5 synthesis.

The onset times and sites of human C1q, C4, C3, and C5 synthesis were determined by culturing tissues from 23 fetuses, 8-25 wk old, in the presence of [(14)C]lysine and isoleucine. In parallel, IgG and IgM production was followed. Liver, spleen, placenta, peritoneal and bone marrow cells, thymus, and colon were cultured for 48 h and the concentrated media studied by immunoelectrophoresis and subsequent autoradiography using adult human serum as carrier and specific antisera. The quantitative synthesis was approximated by scoring the intensity of the labeled precipitin lines using uniform conditions. C5 production was detected earliest at 8 wk gestation and by 11 wk and thereafter, C3, C4, and C5 synthesis was uniformly present in multiple tissues. C1q synthesis, however, was limited almost exclusively to the spleen, began at 14 wk, and was not uniformly present. In contrast IgG and IgM production did not occur in three fetuses synthesizing complement and while detected as early as 11 wk. was inconstant, occurred predominantly in the spleen, and was quantitatively much less compared to C3, C4, and C5. These findings suggest that developmentally the complement system is a more primative biological defense mechanism than antibody.