The body defends itself against foreign proteins and infectious microorganisms by means of a dual system that depends on recognising a of the surface pattern of the invader. The two parts of the system are termed cellular immunity, in which lymphocytes are the effective agent, and humoral immunity, based on the action of antibody molecules.
When lymphocytes recognise a foreign molecular pattern (termed an antigen), some antibodies in great numbers; others store the memory of the pattern for future of antibodies should the molecule reappear. Antibodies themselves to the antigen and in that way mark them for destruction by other substances in the body's defence arsenal. These are complement, a of enzymes that make holes in foreign cells, and phagocytes, cells that engulf and digest foreign matter. They are drawn to the by substances by activated lymphocytes.
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 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 up with the bloodstream through the jugular and subclavian veins; valves in the lymphatic vessels flow in one direction. Nodes at 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 as the vehicle of the immune system was not recognized until the 1960s.