A hypersensitive response by the immune system to a foreign substance that is ordinarily harmless.

Allergies account for more office visits to pediatricians than any other ailment, besides the common cold, and are responsible for more missed school days than any other medical problem. It is thought that at least one out of every five children has an allergy of some kind. Allergies are often inherited—a child with one allergic parent has a 25-35% chance of developing allergies, and the likelihood rises to between 50 and 65% if both parents have allergies. The substances that cause allergies—such as dust or pollen—are known as allergens (or antigens). In persons with special sensitivities, they produce elevated levels of immunoglobulin E (IgE) antibodies, which in turn unleash chemicals called histamines that produce allergic reactions. Common allergens include pollen, animal dander, house dust, chemicals, feathers, and a number of different foods...