What roles do the red bone marrow, the spleen, the thymus, and the lymph nodes play in the body's immune system?
Red bone marrow functions in adults to produce hemopoietic 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 site of maturation 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.
