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A protective response of the body's immune system to an antigen, especially a microorganism or virus that causes disease. The immune response involves the action of lymphocytes that deactivate antigens either by stimulating the production of antibodies (humoral immune response) or by a direct attack on foreign cells (cell-mediated immune response.) An inability to produce a normal immune response results in immunodeficiency diseases such as AIDS.

See also cell-mediated immune response