What causes some people to have allergies?

An allergy is an overreaction of the body's immune defense system to a specific foreign material that would produce no effects in a nonsensitive person. Allergic reactions result from the activity of a specific class of immunologic substance called immunoglobulin E. Allergic reactions can range from mild to serious; from skin rash, runny nose and watering eyes to a life-threatening loss of blood pressure and state of shock. It is not known why a person may be allergic to a particular substance, but susceptibility to allergies in general seems often to be inherited. Each allergic person may have a unique ''allergic fingerprint,'' a group of substances to which the individual may have allergic reactions. There is a widespread but unproved belief that infection in early childhood may sometimes touch off allergic disease, particularly asthma. Although an allergy may last throughout a person's life, certain allergies that develop in childhood may be outgrown, while others can develop later in life, after repeated exposure to the allergy-provoking substance.