a complex system in vertebrates that helps protect the body against pathological effects of foreign substances (antigens), such as viruses and bacteria. The organs involved include the bone marrow and thymus, in which lymphocytes—the principal agents responsible for specific immune responses—are produced, together with the spleen, lymph nodes, and other lymphoid tissues and various chemicals (e.g., cytokines) that mediate the immune response. The immune system interacts with both the nervous system and the endocrine system. See also psychoneuroimmunology.
n. an inflammation of the pancreas, marked by severe abdominal pain and caused by biliary tract disorders (e.g., gallstones), alcoholism, viral infection, or reactions to certain drugs (e.g., some antipsychotic agents).