Understanding The Immune System

Cells of the immune system

Additional lymphocytes reach the lymph nodes (and other immune tissues) through the bloodstream. Each node is supplied by an artery and a vein; lymphocytes enter the node by traversing the walls of very small specialized veins. All lymphocytes exit lymph nodes in lymph via outgoing lymphatic vessels. Much as small creeks and streams empty into larger rivers, the lymphatics feed into larger and larger channels. At the base of the neck large lymphatic vessels merge into the thoracic duct, which empties its contents into the bloodstream. Once in the bloodstream, the lymphocytes and other assorted immune cells are transported to tissues throughout the body. They patrol everywhere for foreign antigens, then gradually drift back into the lymphatic vessels, to begin the cycle all over again. The immune system stockpiles a tremendous arsenal of cells. Some staff the general defenses, while others are trained on highly specific targets. To work effectively, however, most immune cells require the active cooperation of their fellows. Sometimes they communicate through direct physical contact, sometimes by releasing versatile chemical messengers. In order to have room for enough cells to match millions of possible foreign invaders, the immune system stores just a few of each specificity When an antigen appears, those few specifically matched cells are stimulated to multiply into a full scale army. Later, to prevent this army from overexpanding wildly like a cancer, powerful suppressor mechanisms come into play. Lymphocytes are small white blood cells that bear the major responsibility for carrying out the activities of the immune system; they number about one trillion. The two major classes of lymphocytes are B cells, which grow to maturity independent of the thymus, and T cells, which are processed in the thymus. Both B cells and T cells recognize specific antigen targets.