Immunological Significance of the Thymus of the Adult Mouse

MICE thymectomized in neonatal life have shown a marked deficiency of small lymphocytes in the peripheral blood, spleen, lymph nodes and Peyer's patches 1–3 and a depression of the immune response to both viral and bacterial antigens 4 and to allogeneic 1,2,4,5 and even heterospecific 2,4 skin grafts. Mice thymectomized after 3 weeks of age have not shown any significant depression of the immune response to foreign skin grafts nor any marked reduction in the lymphocyte population 1,2,4 . These observations have suggested the hypothesis 1 that during embryonal and early neonatal life the thymus produces the progenitors of immunologically competent cells which mature and migrate to other sites; alternatively, it would produce a humoral factor essential for the maturation and proliferation of lymphocytes throughout the body during development. Evidence from this laboratory, summarized below, suggests that the function of the thymus in initiating immunogenesis is not necessarily restricted to early life.