Interactions between the immunological responses of a thymus-independent antigen (Salmonella adelaide O antigen) with a thymus-dependent antigen (sheep erythrocytes) in the adult bird.

The bird's antibody response to a thymus-dependent antigen (sheep erythrocytes) (SRBC) and a thymus-independent antigen (SALMONELLA ADELAIDE O antigen) were characterized: whereas the former proceeded through a brief 19S response to a declining 7S response, the latter failed to switch from 19S TO 7S for several weeks and consisted in repeated excursions of 19S antibodies. When injected intravenously and simultaneously an injection of S. adelaide-killed organisms and SRBC interact, so that the response to the latter fails to switch from 19S TO 7S and consists of repeated excursions of 19S antibodies. The changed character of the SRBC response is interpreted to be due to the relative lack of 7S antibody: passive 7S antibody to S. adelaide O antigen or 7S anti-SRBC produces a negative feedback inhibition of their respective responses, so that only one excursion of 19S antibody is observed. The effect is not, however, symmetrical; the thymus-independent antigen is dominant. Thus, whereas 7S antibody to S. adelaide produces the same negative feedback inhibition on the response to S. adelaide and the response to SRBC (when injected with adlaide), 7S antibody to SRBC inhibits only the response to SRBC and not the response to S. adelaide. These results are discussed relation to current hypotheses of antibody biosynthesis and mechanisms of adjuvant action. They are also discussed in relation to the function of the germinal centres of the spleen which may function to mediate the negative feedback of 7S antibody on the antibody response.