Antigen-binding small lymphocytes in the guinea-pig. II. The immunological response to purified protein derivative of mammalian tuberculin.

A mean of 6.9 per cent of small lymphocytes in peripheral blood preparations and between 1.8 and 2.4 per cent of small lymphocytes in lymph node, spleen, bone marrow and thymus preparations from unimmunized guinea-pigs bound I-labelled purified protein derivative of mammalian tuberculin (mammalian PPD). The percentage of these cells fluctuated but did not alter substantially after immunization with BCG or with BCG emulsified with human thyroglobulin (HTg) in Freund's incomplete adjuvant (FIA). Blocking experiments indicated that the binding of I-labelled mammalian PPD was specific and there was tentative evidence that the lymphocyte receptors may be IgG. A comparison is drawn between the observed time course of I-labelled mammalian PPD-binding small lymphocytes and the response of lymphocytes sensitive to strong histocompatibility antigens, and it is proposed that the propensity of certain antigens to induce a delayed hypersensitivity-type response is related to the presence of substantial numbers of antigen-binding cells in unimmunized animals. A noteworthy incidental finding was an unexplained depression in the cellular and humoral responses to mammalian PPD in guinea-pigs that had been immunized with HTg—BCG—FIA emulsion.