Understanding The Immune System

immune complexes, which become lodged in body tissues and set off inflammatory reactions (see Immune Complex Diseases). Autoimmune diseases affect the immune system at several levels. In patients with SLE, for instance, B cells are hyperactive while suppressor cells are underactive; it is not clear which defect comes first. Moreover, production of IL-2 is low, while levels of gamma interferon are high. Patients with rheumatoid arthritis, who have a defective suppressor T cell system, continue to make antibodies to a common virus, whereas the response normally shuts down after about a dozen days. No one knows just what causes an autoimmune disease, but several factors are likely to be involved. These may include viruses and environmental factors such as exposure to sunlight, certain chemicals, and some drugs, all of which may damage or alter body cells so that they are no longer recognizable as self. Sex hormones may be important, too, since most autoimmune diseases are far more common in women than in men. Heredity also appears to play a role. Autoimmune reactions, like many other immune responses, are influenced by the genes of the MHC. A high proportion of human patients with autoimmune disease have particular histocompatibility types. For example, many persons with rheumatoid arthritis display the self marker known as HLA-DR4. Many types of therapies are being used to combat autoimmune diseases. These include corticosteroids, immunosuppressive drugs developed as anticancer agents, radiation of the lymph nodes, and plasmapheresis, a sort of "blood washing" that removes diseased cells and harmful molecules from the circulation.

Immune Complex Disease               Immunodeficiency Disease

Immune complexes are clusters of interlocking antigens and antibodies. Under normal conditions immune complexes are rapidly removed from the bloodstream by macrophages in the spleen and Kupffer cells in the liver. In some circumstances, however, immune complexes continue to circulate. Eventually they become trapped in the tissues of the kidneys, lung, skin, joints, or blood vessels. Just where they end up probably depends on the nature of the antigen, the class of antibody-IgG, for instance, instead of IgM-and the size of the complex. There they set off reactions that lead to inflammation and tissue damage. Immune complexes work their damage in many diseases. Sometimes, as is the case with malaria and viral hepatitis, they reflect persistent low-grade infections. Sometimes they arise in response to environmental antigens, such as the moldy hay that causes the disease known as farmer's lung. Frequently, immune complexes develop in autoimmune disease (see above), where the continuous production of autoantibodies overloads the immune complex removal system. Lack of one or more components of the immune system results in immunodeficiency disorders. These can be inherited, acquired through infection or other illness, or produced as an inadvertent side effect of certain drug treatments. People with advanced cancer may experience immune deficiencies as a result of the disease process or from extensive anticancer therapy. Transient immune deficiencies can develop in the wake of common viral infections, including influenza, infectious mononucleosis, and measles. Immune responsiveness