The body defends itself against foreign proteins and infectious microorganisms. When lymphocytes, the effective agents of the immune system, recognize a foreign molecular pattern (termed an antigen), they release antibodies in great numbers; other lymphocytes store the memory of the pattern for future release of antibodies should the molecule reappear. Antibodies attach themselves to the antigen and in that way mark them for destruction by other substances in the body’s defense arsenal.Lymphocytes, which resemble blood plasma in composition, are manufactured in the bone marrow and multiply in the thymus and spleen. They circulate in the bloodstream, penetrating the walls of the blood capillaries to reach the cells of the tissues. From there they migrate to an independent network of capillaries that is comparable to and almost as extensive as that of the blood’s circulatory system. The capillaries join to form larger and larger vessels that eventually link up with the bloodstream through the jugular and subclavian veins; valves in the lymphatic vessels ensure flow in one direction. Nodes at various points in the lymphatic network act as stations for the collection and manufacture of lymphocytes; they may become enlarged during an infectious disease. In anatomy, the network of lymphatic vessels and the lymph nodes are together called the lymphatic system; its function as the vehicle of the immune system was not recognized until the 1960s. (Adapted from Encarta Encyclopedia and National Geographic)