The immune system contains nonspecific or innate defenses that guard against many type of pathogen and respond quickly. Adaptive or specific defenses(immunity) respond only against a specific type of pathogen and respond slowly; is accomplished by soecialized lymphocytes which secrete cytokines or antibodies. Mechanical barriers, inflammation,chemical barriers, acidic environments, enzymes, interferons, NK cells, preforins, phagocytosis, macrophages, fever and the spleen all participate in the removal of pathogens. Immunity is a response by the body against specific pathogens, their toxins, or metabolic products. Immunity is performed by lymphocytes and macrophages that recognize and remember specific foreign molecules on particular pathogens. Immunity includes cellular and humoral immune response. Antigens are any large molecule that can trigger an immune response, before birth, the body makes an inventory of "self antigens". Immune response is against "non self" molecules which are usually large and complex foreign molecules. Red bone marrow releases undifferentiated lymphocyte precursors into circulation. About half of those go to the thymus and mature into T cells and the other half stay in the bone marrow and become b cells. T cells become activated when an APC such as B cells or Macrophages which display the antigen fragments on their own cell membrane, with MHC proteins that help T cells recognize displayed antigens. T cells become activated when they recognize and bind to antigenic fragments that match their receptors. Cellular response is through cell to cell contact as activated T cells interact directly with antigen-bearing cells. T cells also synthesize and secrete cytokines. Some T cells secrete toxins, growth inhibiting factors, or interferon. The humoral response is when antibodies travel through the body fluids to attack and destroy antigens . Plasma cells synthesize and secrete antibodies whose molecular structure is similar to the activated B cells antigen receptors. Hypersensitivity is reactions that are excessive immune responses to a normally harmless antigen. Allergy mediators cause mucus production, vasodilation, and bronchoconstriction resulting in a inflammatory response. Allergy mediators sometimes flood the body, resulting in anaphylaxis, a severe form of immediate reaction hypersensitivity; life threatining. Antibody dependant cytotoxic reactions occur in a transfusion reaction to mismatched blood. Delayed reaction hypersensitivity results from repeated exposure of skin to certain substances.Autoimmine disorders in the immune system manufacture antibodies called autoantibodies and cytotoxic T cells against some of its own antigens. Autoantibodies damage the bodys own tissue.