Red bone marrow functions in adults to produce hematopoietic stem cells and also to produce the lymphoid stem cells that give rise to B lymphocytes. The spleen, which produces hemopoietic stem cells in embryos along with the liver, is a major store house of lymphocytes. The thymus is another major storehouse of lymphocytes. The thymus produces T lymphocytes from lymphoid stem cells that travel to the thymus from the red bone marrow after arising from hemopoietic stem cells. The thymus maintains regulatory control over T lymphocytes by means of a hormone it secretes; T lymphocytes, in turn, regulate B lymphocyte responses. Lymph nodes, also, harbor large numbers of lymphocytes as well as large numbers of macrophages. Thus, lymph nodes act as filtration points removing antibody-antigen combinations from solution.
