Role of the thymus in immune reactions in rats. III. Changes in the lymphoid organs of thymectomized rats.

In rats thymectomized at birth, there was a profound depletion of small lymphocytes in various lymphatic organs. In the spleen, these cells were completely lacking from the Malpighian bodies and splenic white pulp. Empty reticular structures remained surrounding the white pulp arterioles. In the lymph nodes, large masses and nodules of small lymphocytes (primary lymphoid nodules) were either markedly depleted or absent, as were the zones of these cells normally surrounding germinal centers. In both spleen and nodes, germinal centers appeared normal in size, number, and cellular make-up; and plasma cells were found in normal or even increased number in their customary position. Rats which in spite of thymectomy developed intense Arthus or delayed reactivity showed incomplete depletion of the lymphoid tissue. It is concluded that small lymphocytes of the spleen and lymph nodes may come, in large part, directly from the thymus and are not derived from medium and large lymphocytes of the germinal centers. It is suggested that there may be a second population of small lymphocytes whose function is unrelated to the thymus lymphocytes.