Anti-thymocyte serum may enhance or suppress the response to the same antigenic determinant.

Treatment of BALB/c mice with heterologous anti-thymocyte serum (ATS) increases their response to the C-polysaccharide of a rough strain of Pneumococcus, R36A, but suppresses their response to a vaccine of the whole organism. Phosphorylcholine is the major antigenic determinant for both the C-polysaccharide and vaccine. ATS also suppresses the response to phosphorylcholine when it is coupled to various protein carriers. In so far as was examined, ATS had no effect on adherent cells or the capability of adherent cells and antigen to stimulate an antibody response in non-adherent cells. The contrasting effects of the antiserum can best be accounted for by assuming that it acts on at least two different populations of thymic-derived cells.