The role of lymphocytes in antibody formation III. The origin from small lymphocytes of cells forming direct and indirect haemolytic plaques to sheep erythrocytes in the rat

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Abstract
The direct Jerne plaque assay was used in a study of the primary response of rats to a single intravenous injection of sheep red blood cells (SRBC). It was found that radiation induced unresponsiveness to this antigen could be corrected by transfusion of normal syngeneic thoracic duct lymphocytes before immunization. The size of the response restored to irradiated recipients was a function only of the numbers of injected small lymphocytes. This suggested that the restorative activity of a thoracic duct lymphocyte population was present only in the small lymphocyte fraction. The immunoglobulin sequence in the response of rats to SRBC was recorded using the indirect plaque assay to reveal cells secreting antibody of low haemolytic efficiency. Normal syngeneic thoracic duct lymphocytes were able to restore the immunoglobulin sequence essentially unchanged in either dimensions or kinetics to irradiated immunized recipients. All the restorative activity for both direct and indirect plaques was found in small lymphocytes, implying that antibody heterogeneity in the immune response is not another reflexion of the physiological differences between large and small lymphocytes. Using allo-antigenically labelled thoracic duct lymphocytes in lethally irradiated recipients it was shown that the restorative activity of small lymphocytes is mediated by their transformation into antibody forming cells. All the radiosensitive cells necessary for the induction and expression of the antibody response to SRBC in rats are present in the thoracic duct and they are all small lymphocytes.