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. Three types: isotopes (variations in heavy chains in the classes), Allotypes (variation in genes from one person to another allele in the various genes, (Idiotypes) variations in antigen binding site determines antigen specificity. IgA: secretory antibody in lymph, saliva, tears, breast milk, prevent pathogen adherence to cells of mucous membranes. IgD: trace amounts in serum, abundant on B cell surface, signal B cell. IgE: response disproportionate to amount, cause anaphylaxis. IgG: activates complement, antigen elimination. IgM: first antibody produced, agglutinates bacteria, activates complement, enhances phagocytosis.