A term commonly, but inaccurately used to describe any adverse reaction by the body to any substance ingested by the affected individual through the lungs, stomach, skin or mucous membranes (such as the lining of the eye). Medically, allergy has a much narrower definition and refers to a reaction incited by an abnormal immunological response to an ALLERGEN, and susceptibility has a strong genetic component. Most allergic disorders are linked to ATOPY, the predisposition to generate the allergic antibody immunoglobulin E (IgE) in response to common environmental agents (see ANTIBODIES; IMMUNOGLOBULINS). Because IgE is able to sensitise MAST CELLS (which play a part in inflammatory and allergic reactions) anywhere in the body, atopic individuals often have disease in more than one organ. Since the allergic disorder HAY FEVER was first described in 1819, allergy has moved from being a rare condition to one afflicting almost one in three people in the developed world, with substances such as grass and tree pollen, house-dust mite, bee and wasp venom, egg and milk proteins, peanuts, antibiotics, and other airborne environmental pollutants among the triggering factors. Increasing prevalence of allergic reactions has been noticeable during the past two decades, especially in young people with Western lifestyles.

Many immune mechanisms also contribute to allergic disorders; however, adverse reactions to drugs, diagnostic materials and other substances often do not involve recognised immunological mechanisms and the term ‘hypersensitivity’ is preferable. (See also IMMUNITY.)

Allergic reactions may show themselves as URTICARIA, wheezing or difficulty in breathing owing to spasm of the BRONCHIOLES, swollen joints, nausea, vomiting or headaches. Severe allergic reactions may cause a person to go into ANAPHYLAXIS. Although symptoms of an allergic reaction can usually be controlled, treatment of the underlying condition is more difficult, so the best approach is for susceptible individuals to identify what it is they are allergic to and avoid those agents. For some people, such as those sensitive to insect venom, IMMUNOTHERAPY or desensitisation can be effective. If avoidance measures are unsuccessful and desensitisation ineffective, the inflammatory reactions can be controlled with CORTICOSTEROIDS, while the troublesome symptoms can be treated with ANTIHISTAMINE DRUGS and SYMPATHOMIMETICS. All three types of drugs may be needed to treat severe allergic reactions.

One interesting idea, called the ‘hygiene hypothesis’ is that reduced exposure to infective agents, such as bacteria, in infancy may provoke the development of allergy in later life. This is supported by experimental evidence that exposure in early infancy to potential allergens, such as cat or dog hair or peanuts, leads to a lower incidence of subsequent allergy to those materials.