Allergy definition

Allergy is an abnormal sensitivity to an ordinarily harmless substance, called an allergen, that produces a hypersensitivity reaction (allergic reaction) in response to the immune system’s detection of the substance’s presence. A person can have an allergy to nearly any substance.

Though researchers understand the mechanisms of hypersensitivity reaction, they do not know what causes the immune system to determine the substance is a potential invader. The first exposure to the substance activates an immune response that stimulates B-lymphocytes (specialized white blood cells) to produce antibodies. Second and subsequent exposures engage the antibody response, in which the antibodies bind with molecules bearing the allergen to mark them for destruction.

Symptoms of Allergy

Allergies are common. Symptoms vary according to the allergen. Some symptoms remain localized, affecting only a distinct part of the body (such as in allergic dermatitis or allergic rhinitis). Others are systemic, affecting the body as a whole (such as hypersensitivity reaction to a drug or a food item). A severe hypersensitivity reaction can cause life-threatening swelling of the THROAT and airways (anaphylaxis).


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