The functions of the thymus.

In rodents the thymus performs at least two functions. It is a major site of lymphopoiesis in the embryo and newborn, with the resulting lymphocytes migrating from the thymus to seed the spleen, lymph nodes and other lymphoid organs. In addition, the thymus produces a hormone which has an immunotrophic effect, i.e. it endows cells having immunological potential with immunological competence. In some animals other organs, in addition to the thymus, are responsible for directing the normal development of the immunological system. These are the bursa of Fabricius in birds and the appendix in rabbits. In humans it has been postulated that the tonsillar tissues may play an analogous role. Animal experiments involving extirpation of the immunotrophic lymphoid tissues have led to a better understanding of immunological deficiency diseases in man.