Exaggerated response to foreign agent, typically leads to damage in the body. Type I: allergic reaction due to IgE production in response to innocuous antigen. IgE binds to mast cells and anaphylaxis in triggered. Food allergies, penicillin, insect venom. Type II: cytotoxic/cytolytic, death or lyse of cells. IgG or IgM-mediated that activates complement pathway and effector cells. Rejection of blood transfusion. Kills cells. Type III: Immune complex formation, this accumulation results in the damage to blood vessels, kidneys and skin. Can be caused by a persistent infection, continued synthesis of autoantibody and repeated inhalation of allergens. Ex is farmers lung. Type IV: delayed type. T cell mediated, T cells migrate to the site, recognize the antigens bound to the tissue, T cells are activated and release cytokines and lymphocytes, macrophages and basophils are recruited. Poison ivy is an example.