Lymphatic System

Of the remaining lymphatic system components, the thymus, bone marrow, spleen, and Peyer's patches have fairly unique roles. Both the bone marrow and thymus introduce "virgin" lymphocyte to the lymphatic system. The spleen filters old rbcs from the blood and fights infections with lymphocytes and monocytes (cells that engulf and devour antigens). And the Peyer's patches are lymph tissue pockets under raised intestinal projections that examine intestinal contents for foreign antigens. Although the spleen's role is important, the human body is capable of functioning without it if it becomes injured or diseased.

Although the thymus is critical for T cell development in children, it begins to shrink as they progress toward adulthood and thereafter plays an increasingly reduced role. T cells are "educated" in the thymus to recognize "self" versus "nonself" (foreign) antigens. Without the ability to recognize self-antigens, T cells would target a person's own tissues in a very destructive manner. The human lymphatic system. Illustration by Argosy. The Gale Group. The thymus is also responsible for fostering maturation of T cells into their various subclasses. T cells function in a cell-mediated way such that they only recognize antigens presented to them by other cells; hence, T cell immunity is called cell-mediated immunity.

Both T cells (before branching off to develop in the thymus) and B cells originate in the pluripotential stem cells of the bone marrow or the fetal liver. Pluripotential stem cells are the body's cellular sculpting clay. They can be shaped into any cell-including lymphocytes, rbcs, macrophages, and numerous other blood constituentsand become increasingly specialized as they reach maturity. The B cells can generate an infinite number of antibodies in response to a multitude of foreign antigens. This amazing diversity arises from the many combinations of antibody components that can be rearranged to recognize individual antigens. Once a B cell identifies a particular enemy, it undergoes a process called clonal expansion. During this process, it makes many clones (copies) of itself in order to fight several invaders of a single type. This highly sophisticated molecular process destroys infections wherever they arise in the body.

One specialized form of antibody, IgA, detects antigens in the gastrointestinal tract at Peyer's patches. IgA contained within small projections, called lamina propriae, that extend into the small intestine test the intestinal lining for pathogens. The IgA binds to the foreign antigen, returns to exit the patch at its efferent lymphatic vessel, and travels to a mesenteric lymph node that gears up to fight the invader. IgA antibodies are also passed to nursing babies in their mothers' milk, because newborns do not synthesize IgAs until later.