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A state wherein the immune mechanisms are inadequate in their ability to perform their normal function, that is, the elimination of foreign materials (usually infectious agents such as bacteria, viruses, and fungi). Immune mechanisms are also responsible for the rejection of transplanted organs. These processes are accomplished by white blood cells known as lymphocytes, of which there are two major types, T lymphocytes (thymus-derived) and B lymphocytes (bone marrow-derived). See Cellular immunology, Transplantation biology
Immunological deficiency states result from a failure at any point in the complex set of interactions involving lymphocytes and immunoglobulins. In general, the diseases are due to absence of cell populations; failure of cells to mature; failure to secrete the products necessary for effective cell interactions; or failure of accessory cell populations or protein systems (for example, complement) which are necessary for the complete competence of B-cell immune function. Some of the diseases are carried on the X-chromosome and affect only males, being carried by females. See Complement, Sex-linked inheritance
The prime symptom of immunodeficiency is an increased suceptibility to infections. Many of the organisms to which people are constantly exposed do not ordinarily have the capability to cause infections in immunocompetent individuals because these organisms are so weak that they cannot establish themselves in normals. In immunodeficients, however, they can cause fatal infections.
In general, immunodeficiency states are inherited. Immunodeficiency can also be acquired as a complication of other disease processes. One of the most common forms of deficiency is caused by aggressive treatment of leukemia. Another cause of induced immunodeficiency is seen with transplant rejection therapy. To prevent organ rejection, drugs which destroy lymphocytes must be administered. Certain viruses, such as the Epstein-Barr virus (EBV), which causes infectious mononucleosis, infect lymphocytes. Involvement of the lymphoid system is nearly always only temporary, but in a small number of individuals the virus cannot be eliminated, and a chronic infection of B cells leads to the loss of normal lymphocyte function. Another immunodeficiency disorder caused by virus infection is acquired immune deficiency syndrome (AIDS). Immunodeficiency can also be observed secondary to dietary deficiency. Two main varieties are seen. In protein-calorie malnutrition, serious deficiency primarily involving the T-cell system predisposes affected individuals to overwhelming infection by the agents of measles or tuberculosis. In the second variety, deficiency of single substances is the cause; the two most commonly observed deficiencies are those of zinc and biotin. See Acquired immune deficiency syndrome (AIDS), Transplantation biology