The immune system is made up of lymphoid tissue in the body, which includes the bone marrow, lymph nodes, thymus, tonsils, and parts of the spleen and gastrointestinal tract. The immune system helps protect the body from harmful substances called antigens . Examples of antigens include bacteria, viruses, toxins , cancer cells, and foreign blood or tissues from another person or species. When the immune system detects an antigen, it responds by producing antibodies that destroy the harmful substances. The immune system response also involves a process called phagocytosis. During this process, certain white blood cells swallow and destroy bacteria and other foreign substances. Immune system disorders occur when the immune system does not fight tumors or harmful substances as it should. The immune response may be overactive or underactive. Immunodeficiency disorders may affect any part of the immune system. Most commonly, such a condition occurs when specialized white blood cells called T or B lymphocytes (or both) do not work as well as they should, or when your body doesn't produce enough antibodies. Inherited immunodeficiency disorders that affect B cells include: Hypogammaglobulinemia, which usually causes mild to moderate respiratory infections; Agammaglobulinemia , which results in frequent severe infections and is often deadly. Inherited immunodeficiency disorders that affect T cells may cause increased susceptibility to fungi, resulting in recurring Candida (yeast) infe
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