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 molecule. It is believed that 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.
The antibodies produced by B cells are a type of globulin protein called immunoglobulins. There are five classes of immunoglobulins designated IgA, IgD, IgE, IgG, and IgM; gamma globulin (IgG) predominates. Antibody molecules are able to chemically recognize surface portions, or epitopes, of large molecules that act as antigens, such as nucleic acids, proteins, and polysaccharides. About 10 amino acid subunits of a protein may compose a single epitope recognizable to a specific antibody. The fit of an epitope to a specific antibody is analogous to the way a key fits a specific lock. The amino acid sequence and configuration of an antibody were determined in the 1960s by the biochemists Gerald Edelman, an American, and R. R. Porter, an Englishman; for this achievement they shared the 1972 Nobel Prize for Physiology or Medicine.
The antibody molecule consists of four polypeptide chains, two identical heavy (i.e., long) chains and two identical light (i.e., short) chains. All antibody molecules are alike except for certain small segments that, varying in amino acid sequence, account for the specificity of the molecules for particular antigens. In order to recognize and neutralize a specific antigen, the body produces millions of antibodies, each differing slightly in the amino acid sequence of the variable regions; some of these molecules will chemically fit the invading antigen.
Antibodies act in several ways. For example, they combine with some antigens, such as bacterial toxins, and neutralize their effect; they remove other substances from circulation in body fluids; and they bind certain bacteria or foreign cells together, a process known as agglutination. Antibodies attached to antigens on the surfaces of invading cells activate a group of at least 11 blood serum proteins called complement, which cause the breakdown of the invading cells in a complex series of enzymatic reactions. Complement proteins are believed to cause swelling and eventual rupture of cells by making holes in the lipid portion of the cell's membrane.
After their production in the bone marrow, some lymphocytes (called T lymphocytes or T cells) travel to the thymus, where they differentiate and mature. The T cells interact with the body's own cells, regulating the immune response and acting against foreign cells that are not susceptible to antibodies in what is termed "cell-mediated immunity." Three classes of T lymphocytes have been identified: helper T cells, suppressor T cells, and cytotoxic T cells. Each T cell has certain membrane glycoproteins on its surface that determine the cell's function and its specificity for antigens.
One type of function-determining membrane glycoprotein exists in two forms called T4 or T8 (CD4 or CD8 in another system of nomenclature); T4 molecules are on helper T cells, T8 molecules are on suppressor and cytotoxic T cells. Another type of membrane glycoprotein is the receptor that helps the T cell recognize the body's own cells and any foreign antigens on those cells. These receptors are associated with another group of proteins, T3 (CD3), whose function is not clearly understood. T cells distinguish self from nonself with the help of antigens naturally occurring on the surface of the body's cells. These antigens are, in part, coded by a group of genes called the major histocompatibility complex (MCH). Each person's MCH is as individual as a fingerprint.
When a cytotoxic T lymphocyte recognizes foreign antigens on the surface of a cell, it again differentiates, this time into active cells that attack the infected cells directly or into memory cells that continue to circulate. The active cytotoxic T cells can also release chemicals called lymphokines that draw macrophages. Some (the "killer T cells") release cell-killing toxins of their own; some release interferon. Helper T cells bind to active macrophages and B lymphocytes and produce proteins called interleukins, which stimulate production of B cells and cytotoxic T cells. Although poorly understood, suppressor T cells appear to help dampen the activity of the immune system when an infection has been controlled.