Bone marrow (Primary lymphoid organ)

Bone marrow is the soft material in the cavities of bones. It is a network of connective tissue fibers, fat cells, blood vessels, and blood-producing cells. Bone marrow produces both red and white blood cells, including the lymphocytes. Both T-lymphocytes and B-lymphocytes are produced in the bone marrow. The young T-cells move to the thymus for final development, but the B-cells remain in the bone marrow during maturation. Once the B-cells are fully developed in the bone marrow, they are also released into circulation and most of them take up residence in the secondary lymphatic organs.

The B-cells are white blood cells that are sensitive to antigens and produce antibodies against them. Antigens are any chemicals that produce an immune response in the body, such as toxins, foreign proteins, particulate matter, or bacterial cells. When an antigen is present, the B-cell becomes active and begins to produce antibodies against that antigen. Antibodies are special proteins that bind (attach) to antigens and mark them for destruction. Antibodies are antigen specific, and the immune system is able to remember each antigen it fights. Once a B-cell makes antibodies against a certain antigen, e.g., a bacteria, it keeps a memory of that antigen. If the antigen appears again, the B-cell can produce a large number of antibodies very rapidly. In this way, a second infection with that bacteria is often prevented.