Hypersensitivity

Overreaction of the immune system to an antigen. There are 4 main types of hypersensitivity. Type I is associated with allergy. After a first exposure to an antigen, antibodies are formed; these coat cells called mast cells in various tissues. On second exposure, the antigen and antibodies combine, causing the mast cells to disintegrate and release chemicals that cause the symptoms of asthma, allergic rhinitis, urticaria, anaphylactic shock, or other allergic illnesses. In type II reactions, antibodies that bind to antigens on cell surfaces are formed, leading to possible destruction of the cells. Type II reactions may lead to certain autoimmune disorders. In type III reactions, antibodies combine with antigens to form particles called immune complexes. These lodge in various tissues and activate further immune system responses, leading to tissue damage. This type of reaction is responsible for serum sickness and for the lung disease allergic alveolitis. In type IV reactions, sensitized T-lymphocytes (a class of white blood cell) bind to antigens and release chemicals called lymphokines, which promote an inflammatory reaction. Type IV reactions are responsible for contact dermatitis and measles rash; they may also play a part in “allergic” reactions to drugs. Treatment of hypersensitivity depends on the type, cause, and severity. When possible, exposure to the offending antigen should be avoided.