immune system, a system of tissues, organs, and cells that protects the body against pathogenic organisms and other foreign bodies. The principal components of the immune system include the primary lymphoid tissues (bone marrow, thymus) and the secondary lymphoid tissues (lymph nodes, spleen, tonsils, mucosa-associated lymphoid tissue [MALT]). The immune system protects the body initially by creating local barriers and inflammation. The local barriers provide chemical and mechanical defenses through the skin, the mucous membranes, and the conjunctiva. Inflammation draws polymorphonuclear leukocytes and neutrophils to the site of injury, where these phagocytes engulf the invading pathogens (innate immunity). If these first-line defenses fail or are inadequate to protect the body, the humoral immune response and the cell-mediated immune response are activated (adaptive immunity). See also immune response, humoral immunity, cell-mediated immunity.
