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Immunology => Histocompatibility Groups => Autoimmune Diseases
Autoimmune Diseases
Autoimmune Diseases, illnesses in which the immune system reacts to normal components of the body as if they were foreign substances and produces antibodies against them.
Antibodies are manufactured by cells called lymphocytes. It is thought that, during development of the embryo, lymphocytes capable of reacting with the body's own tissues are somehow inactivated so that the self is distinguished from the nonself and is not destroyed by antibodies. Self-reactive lymphocytes can still be found in some adults, however, which suggests that they are actively suppressed in some way rather than eliminated. Another mechanism for protecting components of the self from destruction is sequestration during early development. Mature sperm cells, for example, do not appear until after the immune system has matured and are then automatically separated from the bloodstream. After vasectomy, these cells enter the bloodstream, where they can provoke formation of antibodies against themselves (autoantibodies).
One theory to explain autoimmune disease proposes that suppression of reaction against the self is disrupted when viruses infect the antibody-forming cells. In the infectious form of mononucleosis, in which lymphocytes are invaded by a virus, antibodies against a variety of body tissues are found in the bloodstream. Rheumatic heart disease is believed to be a result of childhood infection by streptococcal bacteria, which have a surface molecular arrangement identical to one found in heart muscle; antibody formed against the bacteria can also damage the heart.
In most other autoimmune diseases the cause of antibody formation is unknown. Persons with myasthenia gravis make an antibody that blocks the transmission of nerve impulses to muscle; this causes the muscle weakness and breathing difficulty associated with the disease. In autoimmune hemolytic anemia, the red blood cells are destroyed by autoantibodies. Persons with lupus erythematosus make antibodies that attack cell components, including genetic material. Clumps of matter formed by antibodies bound to the cell components can damage the kidneys. The blood of some persons with arthritis contains rheumatoid factor, an antibody that binds to other antibodies in the blood; whether this factor also causes the joint injury of arthritis is not known.
Lupus, arthritis, and the skin diseases scleroderma and dermatomyositis are called collagen diseases because of the damage the associated antibodies cause to connective tissue, which is made up of collagen.
A few severe cases of diabetes are caused by an antibody that destroys insulin-forming cells in the pancreas. Another antibody attacks the thyroid gland, producing chronic thyroiditis. Addison's disease in some cases may result from autoimmune destruction of the adrenal gland.
One of the most intensively studied autoimmune diseases is multiple sclerosis. In this illness the myelin sheath covering the spinal cord is destroyed, leading to difficulty in walking and other movements. The damage in multiple sclerosis is not produced by an autoantibody but by a lymphocyte that reacts directly with the protective sheath.
Autoantibodies are often found in the blood of older persons who have no disease, a phenomenon that is not understood.
Treatment of autoimmune diseases usually entails therapy with steroids , which suppress the immune system. Currently under investigation is a procedure called plasmapheresis, in which the patient's blood is passed through a machine that removes gamma globulin, the blood fraction that contains antibodies.
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