They are generally referred to as lymphoid organs because they are concerned with the growth, development, and deployment of lymphocytes, the white cells that are the key operatives of the immune system. Lymphoid organs include the bone marrow and the thymus, as well as lymph nodes, spleen, tonsils and adenoids, the appendix, and clumps of lymphoid tissue in the small intestine known as Peyer's patches. The blood and lymphatic vessels that carry lymphocytes to and from the other structures can also be considered lymphoid organs. Cells destined to become immune cells, like all other blood cells, are produced in the bone marrow, the soft tissue in the hollow shafts of long bones. The two major classes of lymphocytes are B cells and T cells.
B cells complete their maturation in the bone marrow.
T cells, on the other hand, migrate to the thymus, a multilobed organ that lies high behind the breastbone. There they multiply and mature into cells capable of producing immune response-that is, they become immunocompetent. Upon exiting the bone marrow and thymus, some lymphocytes congregate in immune organs or lymph nodes. They use the blood circulation as well as a bodywide network of lymphatic vessels similar to blood vessels. Thus, the lymph node brings together the several components needed to spark an immune response.