The lymphatic system is commonly divided into the primary lymphoid organs, which are the sites of B and T cell maturation, and the secondary lymphoid organs, in which further differentiation of lymphocytes occurs. Primary lymphoid organs include the thymus, bone marrow, and fetal liver and, in birds, a structure called the bursa of Fabricius. In humans the thymus and bone marrow are the key players in immune function. All lymphocytes derive from stem cells in the bone marrow. Stem cells destined to become B cells remain in the bone marrow as they mature, while prospective T cells migrate to the thymus to undergo further growth. Mature B and T cells exit the primary lymphoid organs and are transported via the bloodstream to the secondary lymphoid organs, where they become activated by contact with foreign materials, or antigens.
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Lymphoid organs