hypersensitivity, heightened response in a body tissue to an antigen or foreign substance. The body normally responds to an antigen by producing specific antibodies against it. The antibodies impart immunity for any later exposure to that antigen. When exposure takes place under certain physiological conditions, or in allergic individuals with abnormal immune systems, a heightened immune response results that causes cell damage. Histamines, substances released from damaged cells, cause dilation of small blood vessels, tissue inflammation, and constriction of the bronchi of the lungs. Anaphylaxis is the immediate, sometimes fatal hypersensitivity reaction to drugs or serum to which an individual has been previously sensitized. Serum sickness is a similar but milder hypersensitivity to serum proteins or drugs that occurs several weeks after injection of foreign material. Delayed reaction allergies occur when cells of the immune system, the lymphocytes, that have previously been sensitized react to antigenic substance. The lymphocytes slowly infiltrate an area, such as skin exposed to poison ivy toxin and cause tissue damage. Anaphylaxis, serum sickness, and delayed sensitivity may occur in otherwise normal, nonallergic individuals as well as allergics, as a response to substances that are highly sensitizing. Individuals with allergic, or atopic, hypersensitivity form special weak types of antibodies, that cause local tissue damage and such symptoms as hives, hay fever, and asthma. Antihistamines are drugs that prevent histamine from acting on blood vessels, bronchioles, and other organs. Acute reactions, such as anaphylaxis, are treated by giving epinephrine and other sympathomimetic drugs. Steroids such as cortisone are also given to suppress inflammation and depress the immune system. In some cases, hypersensitized individuals receive injections of gradually increasing quantities of the antigenic material to which they are sensitive, in order to avoid or lessen their hypersensitivity to that particular substance.