Serum-treated antigen-antibody complexes inhibit the production of C2 and factor B by mononuclear phagocytes.

Antigen-antibody complexes enhanced the synthesis of C2 and factor B by human monocytes and macrophages, and C2 by guinea-pig macrophages. In contrast complexes that had been treated with serum inhibited the production of these components. The inhibitory effect of serum-treated complexes was abrogated by Fab fragments of anti-C3, anti-C3c and anti-C3d. It is therefore probable that inhibition was mediated by a C3 fragment bound to the complex. The enhancing effect of untreated complexes was reversible by serum-treated complexes, and the inhibitory action of serum-treated complexes was counteracted by untreated complexes. Such a system may be important in the regulation of the synthesis of complement components in response to local requirements.