Lymphoid cells from normal animals from two genetically alien but isogenic strains of mice or rats elaborated a factor into the medium during in vitro cultivation. Similarly, lymphoid cells from specifically presensitized animals mixed and cultivated with donor-type lymphoid cells, as well as normal or specifically immunized mouse spleen cells exposed to heterologous red blood cell antigens, formed this factor. Culture supernatants of genetically identical lymphoid cells lacked activity.

This factor was measured by its ability to induce accumulations of polymorphonuclear cells following injection into the skin of normal hamster hosts. Gentle trypsinization of excised 24 hr cutaneous reactions permitted quantitative estimation of the number of polymorphonuclear cells accumulated and, therefore, of the activity of the factor.

The factor present in culture fluids could be defined as a product of antigenic recognition (PAR). PAR was formed only when immunologically competent cells were exposed to cells carrying histocompatibility antigens of recognizable foreignness. This view was based chiefly on the observation that, whereas lymphoid cells from specifically tolerant mice failed to recognize the paralytogen, they did recognize antigens of an unrelated mouse strain. In addition, immunocompetence was of paramount importance, since cocultivation of immunologically incompetent, yet strongly incompatible, rat thymocytes did not result in the formation of PAR.

Formation of PAR was observed in very simple media. A source of energy and a suitable culture vessel were found to be the only essential nonimmunological factors for PAR to be elaborated. The presence of inhibitors of glycolysis in the medium during cultivation of mixtures of immunocompetent cells of disparate origin severely inhibited the recognition process. With cell mixtures of the immunological one-way variety, treatment of aggressor cells with inhibitors of glycolysis or of protein synthesis interfered with antigenic recognition, whereas similar treatments of the target cells were without effect.

The findings presented sustain the thesis that the elaboration of PAR is a very early event in the immune process.

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