IMMUNE SYSTEM DISORDER

any of various failures in the body's defense mechanisms against infectious organisms (see table). Disorders of immunity include immune deficiency diseases, such as AIDS, that arise because of a diminution of some aspect of the immune response. Other types of immune disorders, such as allergies and autoimmune disorders, are caused when the body develops an inappropriate response to a substanceeither to a normally harmless foreign substance found in the environment, in the case of allergies, or to a component of the body, in the case of autoimmune diseases. Lymphocytes (white blood cells of the immune system) can become cancerous and give rise to tumours called leukemias, lymphomas, and myelomas. This article discusses various immune deficiencies, allergies, autoimmune disorders, and lymphocyte cancers. For additional information on leukemias, lymphomas, and myelomas, see the article cancer. A discussion of how the immune system works to prevent disease is found in the article immune system. Additional reading Lloyd J. Old, Immunotherapy for Cancer, Scientific American, 275(9):136143 (September 1996), explores the use of the human immune system in combating cancer. Clinical aspects are covered by G.L. Asherson and A.D.B. Webster, Diagnosis and Treatment of Immunodeficiency Diseases (1980); Max Samter (ed.), Immunological Diseases, 4th ed., 2 vol. (1988); and P.J. Lachmann et al. (eds.), Clinical Aspects of Immunology, 5th ed., 3 vol. (1993). Mechanisms of the immune system Specific, acquired immunity It has been known for centuries that persons who have contracted certain diseases and survived generally do not catch those illnesses again. The Greek historian Thucydides recorded that, when the plague was raging in Athens during the 5th century BC, the sick and dying would have received no nursing at all had it not been for the devotion of those who had already recovered from the disease; it was known that no one ever caught the plague a second time. The same applies, with rare exceptions, to many other diseases, such as smallpox, chicken pox, measles, and mumps. Yet having had measles does not prevent a child from contracting chicken pox, or vice versa. The protection acquired by experiencing one of these infections is specific for that infection; in other words, it is due to specific, acquired immunity, also called adaptive immunity. There are other infectious conditions, such as the common cold, influenza, pneumonia, and diarrheal diseases, that can be caught again and again; these seem to contradict the notion of specific immunity. But the reason such illnesses can recur is that many different infectious agents produce similar symptoms (and thus the same disease). For example, more than 100 viruses can cause the cluster of symptoms known as the common cold. Consequently, even though infection with a particular agent does protect against reinfection by that same pathogen, it does not confer protection from other pathogens that have not been encountered. Acquired immunity is dependent on the specialized white blood cells known as lymphocytes. This section describes the various ways in which lymphocytes operate to confer specific immunity. Although pioneer studies were begun in the late 19th century, most of the knowledge of specific immunity has been gained since the 1960s, and new insights are continually being obtained. The nature of lymphocytes General characteristics Location in the lymphatic system The human lymphatic system, showing the lymphatic vessels and lymphoid organs. Lymphocytes are the cells responsible for the body's ability to distinguish and react to an almost infinite number of different foreign substances, including those of which microbes are composed. Lymphocytes are mainly a dormant population, awaiting the appropriate signals to be stirred to action. The inactive lymphocytes are small, round cells filled largely by a nucleus. Although they have only a small amount of cytoplasm compared with other cells, each lymphocyte has sufficient cytoplasmic organelles (small functional units such as mitochondria, the endoplasmic reticulum, and a Golgi apparatus) to keep the cell alive. Lymphocytes move only sluggishly on their own, but they can travel swiftly around the body when carried along in the blood or lymph. At any one time an adult human has approximately 2 1012 lymphocytes, about 1 percent of which are in the bloodstream. The majority are concentrated in various tissues scattered throughout the body, particularly the bone marrow, spleen, thymus, lymph nodes, tonsils, and lining of the intestines, which make up the lymphatic system (see illustration). Organs or tissues containing such concentrations of lymphocytes are termed lymphoid. The lymphocytes in lymphoid structures are free to move, although they are not lying loose; rather, they are confined within a delicate network of lymph capillaries located in connective tissues that channel the lymphocytes so that they come into contact with other cells, especially macrophages, that line the meshes of the network. This ensures that the lymphocytes interact with each other and with foreign materials trapped by the macrophages in an ordered manner.