Recirculation of Lymphocytes*

The long-lived small lymphocyte population of the rat includes approximately 90 per cent of the cells which appear in thoracic duct lymph. About two-thirds of the small lymphocytes in blood are long-lived. These long-lived cells are produced at a rate proportional to body growth and recirculate from blood to lymph. Althouohtthough a major percentage of the small lymphocytes in lymph nodes and a smaller percentage of those in spleen are of the long-lived variety, the thymus may be their primary source of origin. The small lymphocytes in the bone marrow, approximately 95 per cent of those in the thymus and a major percentage of those in the spleen are short lived (five days or less). Evidence is presented which suggests that the long-lived lymphocyte plays an active role in immune response mechanisms. (Author)