An allergy is a reaction to a foreign substance by a person sensitive to that substance. Allergies are part of the body's immune mechanisms. They are triggered when an antigen--a substance foreign to the body--enters or contacts the body, which stimulates the production of antibodies. When antigens and antibodies interact, they cause the release of active substances into the body fluids. These substances, the most common of which is histamine, cause allergic responses such as muscle contraction, swelling, itching, coughing, and difficulty in breathing. Substances that act as antigens--like eggs, roses, or pet hair--are usually harmless to nonallergic persons.

Allergic reactions have been noted since ancient times, but recognition that they were triggered by external agents did not begin to develop until the sixteenth century. Leonardo Botallo (1530-1575) described the "rose cold" or "rose fever" in 1565, a condition involving asthma and common cold symptoms caused by contact with...