Induction in vivo of enhancing factors for antibody production: II. Ig-Antigen complexes formed within 6 hours after immunization markedly enhance antibody formation

Serum collected 6 hr after injection of SRBC contains helper factor(s) which enhances antibody formation. The factor which markedly enhances the 7s response is an Ig-Ag complex. The complexes contain Ia antigens as shown by the ability of proper immunoadsorbents to remove both the antibody-enhancing activity for 7s, as well as the “cytophilic Ig” which is taken up by T cells. The enhancement of 19s is due either to a different factor which contains neither Ig nor antigen, or to the same Ig-Ag complexes which act at lower concentrations. Both 19s and 7s enhancing activities show no H₂ restriction in the presence of T cells. However no antibody enhancement was obtained when allogeneic 6HS was injected in thymectomized, lethally irradiated, and bone-marrow-reconstituted Balb/c mice, which suggests that the presence of T cells overcomes the H₂ barrier which exists when the complexes act directly on B cells. Since the complexes are formed through the mediation of a T-cell factor they may be considered a T-cell replacing factor and are predominantly concerned with the regulation of 7s response.