Passive immunization of rats with homologous anti-sheep erythrocyte serum markedly inhibited the primary antibody response to various doses of sheep erythrocytes. Inhibition was "specific" and apparently produced by either "19S" or "7S" antibody to the antigen. Passive immunization inhibited splenic hyperplasia associated with the primary antibody response. Passive immunization 24 hours after active immunization effectively inhibited the primary antibody response.
The markedly suppressive effect of specific antibody on the primary antibody response contrasted sharply with the absence of this effect on the secondary response. Antigen-antibody complexes formed in vitro elicited no measurable primary antibody response but did elicit a high secondary response. Exposure of normal spleen cells to the antibody in vivo or in vitro suppressed their response to the antigen in x-irradiated recipients. In contrast, cells from previously immunized animals transferred to x-irradiated animals produced antibody in the presence of passively given antibody. Thus, "potential antibody-forming cells" from normal animals were unresponsive to the antigen in the presence of specific antibody, while "antibody-forming cells" from previously immunized animals responded to the antigen in the presence of antibody. Presumably, antibody actively produced in small quantities by a few antibody-forming cells might inhibit antibody formation by potential antibody-forming cells. Confirmation of this suggestion was obtained by showing that some animals initially injected with small doses of antigen failed to produce measurable antibody to subsequent injections of larger doses of the antigen. Low doses of antigen capable of inducing unresponsiveness produced no measurable circulating antibody, but these doses did produce increased numbers of plaque-forming (antibody-releasing) cells in spleens of rats. Thus, the formation of specific antibody may provide a homeostatic or "feed-back" mechanism which controls or limits production of specific antibody to the portion of the antibody-forming system previously stimulated by the antigen. This mechanism may account in part for immunological unresponsiveness produced in certain other related experimental systems.
HOMEOSTASIS OF ANTIBODY FORMATION IN THE ADULT RAT
