The original concept of allergy included all immune-mediated disease and the term allergy was interchangeable with the term "hypersensitivity." A clinical immunology text will take the approach that allergy and autoimmune disease are the two major categories of hypersensitivity disease.

Allergy can be thought of as hypersensitivity disorders with external causes. Substances which trigger allergic responses are antigens, often proteins, that can be found in air, food and water.

Airborne antigens such as plant pollens or house dust are well known.  Other airborne antigens and food antigens are less obvious. New and foreign substances introduced to the body such as airborne chemicals, drugs and herbs cause allergic reactions as the body identifies foreign antigens and then attempts to get rid of the alien.

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