Allergy

n. excessive sensitivity to certain substances (i.e. animal hair or pollen) which often causes the development of respiratory problems or other symptoms

Allergy is an abnormal reaction to a substance foreign to the body that is acquired, predictable and rapid. In the strict sense of its meaning, it is the first (type 1) of five forms of hypersensitivity described by Gell and Coombs in their 1963 classification. However, by extension, the term "allergy" is often used for other abnormal reactions to substances. The term was coined by the Viennese pediatrician Clemens von Pirquet in 1906 after noting that some of his patients were hypersensitive to normally innocuous entities such as dust, pollen, or certain foods. Pirquet called this phenomenon "allergy", from the Greek words allos meaning "other" and ergon meaning "work".
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Allergy /al·ler·gy/ a hypersensitive state acquired through exposure to a particular allergen, reexposure bringing to light an altered capacity to [more]Allergy - Community and Resources

Noun
1. hypersensitivity reaction to a particular allergen; symptoms can vary greatly in intensity
(synonym) allergic reaction
(hypernym) hypersensitivity reaction
(hyponym) food allergy

A hypersensitivity of the body's immune system in response to exposure to specific substances (antigens), such as pollen, beestings, poison ivy, drugs, or foods. Anaphylactic shock is a severe form of allergy response which is a medical emergency. Symptoms of anaphylactic shock include dizziness, loss of consiousness, labored breathing, swelling of the tongue and breathing tubes, blueness of the skin, low blood pressure, and death.