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The principal parts of the immune system are the bone marrow, thymus, lymphatic system, tonsils, and spleen. The lymph nodes, tonsils, and spleen act to trap and destroy antigens from the lymph, air, and blood, respectively. Antigens are molecules that the body reacts to by producing antibodies, highly specific proteins also known as immunoglobulins. Antigens include bacteria and their toxins, viruses, malignant cells, foreign tissues, and the like. Their destruction is accomplished by white blood cells (lymphocytes and the granulocytes and monocytes mentioned above), which are produced and constantly replenished by the stem cells of the bone marrow. The two types of lymphocytes are called B lymphocytes (B cells) and T lymphocytes (T cells). B cells are responsible for production of antibodies in what is called "humoral" immunity after the ancient medical concept of the body humors.
B Lymphocytes
The presence of antigens in contact with receptor sites on the surface of a B lymphocyte stimulates the lymphocyte to divide and become a clone (a line of descendant cells), with each cell of the clone specific for the same antigen. Some cells of the clone, called plasma cells, secrete large quantities of antibody; others, called memory cells, enter a resting state, remaining prepared to respond to any later invasions by the same antigen. Antibody secretion by lymphocytes can be stimulated or suppressed by such variables as the concentration of antigens, the way the antigen fits the lymphocyte's receptor regions, the age of the lymphocyte, and the effect of other lymphocytes.
According to the modified clonal selection theory originally postulated by the Australian immunologist Sir Macfarlane Burnet (for which he was awarded the 1960 Nobel Prize for Physiology or Medicine), a lymphocyte is potentially able to secrete one particular, specific humoral, or free-circulating, antibody molehat early in life lymphocytes are formed to recognize thousands of different antigens, including a group of autoimmune lymphocytes, i.e., cells recognizing antigens of the organism's own body. The immune system is self-tolerant; i.e., it does not normally attack molecules and cells of the organism's own body, because those lymphocytes that are autoimmune are inactivated or destroyed early in life, and the cells that remain, the majority, recognize only foreign antigens. Burnet's theory was confirmed with the development of monoclonal antibodies.
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