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Allergy and Hypersensitivity

Allergies and hypersensitivity are harmful and unwanted reactions of the body's immune system to ordinarily harmless substances such as pollen, house dust and certain foods. In the same way that an infective organism carries antigens that cause antibodies to be produced, the allergic substance (allergen) also stimulates antibody production. Other reactions occur as well, and many of these are caused by body cells producing potent chemicals with wideranging effects. One of the best-known of these chemicals is histamine, which causes blood vessels to dilate (leading to redness of the skin), smooth muscle to contract, and fluid to be produced by cells, resulting in tissue swelling. These sorts of reaction can affect organs in different ways. There may be swelling in the throat, interfering with breathing; the mucous membranes in the nose also swell and produce fluid. In the skin, a common reaaion is the production of weals (nettle rash or urticaria).

In order to become allergic to a substance a person must be exposed to it a first time when no allergic response occurs. It is only on a second or subsequent exposure that the allergy becomes obvious.

Anaphylaxis is a particular type of severe allergic reaction which can be life-threatening, since the lungs may be so badly affeaed that asphyxiation occurs; immediate medical attention is essential. Anaphylatic shock can occur as a : response to certain drugs, such as aspirin and penicillin, and some foods, but fortunately is rare.

There are many different causes of allergy. Allergy to foods is common, with eggs, cow's milk, shellfish, cereals, vegetables, fruits and alcoholic drinks being particularly allergenic. Certain metals, nickel and chromium in particular, can affect the skin, causing allergic dermatitis, as can some chemicals and drugs. Aspirin can also cause worsening of asthma symptoms, while other drugs may cause either widespread rash or affect only a small area, which erupts in a rash in the same place each time the drug is given. Some people are allergic to substances they encounter at their place of work: for example, hairdressers handling chemicals may suffer dermatitis on the hands, builders may become allergic to cement and bakers may react to sugar-and bread-additives. Allergy to insect bites, plants and pollen also occurs.

Diagnosis and treatment

Although it is relatively simple to determine that a person has an allergy, it is often difficult to identify the particular substance responsible. Sometimes the allergy can be associated with eating unusual food or taking part in a certain activity. In other cases, if the allergen is common, it may be more difficult to identify.

Skin tests are widely used to identify allergens. There are two main types. In scratch tests the skin, normally on the forearm, is scratched or pricked and a solution of each different suspected allergen is placed on each scratch. A positive reaction - a red lump at the site of the scratch - will occur within twenty minutes if the person is allergic to that particular substance. With patch tests, the second type of skin test, different suspected allergens are held against the unbroken skin for up to forty-eight hours and the reaction examined. With both tests, if a reaction occurs it does not necessarily mean that this is the substance causing the problem, and further investigations may be needed.

Avoiding the substance that causes the allergy is one of the best forms of treatment. This can involve a change of diet, employment or residence; removal of pets or articles of furniture, or avoiding certain clothing or cosmetics.

Desensitization is another form of treatment. It consists of a series of injections, containing gradually increasing amounts of the allergen, given over several weeks or months and often repeated for three years. However, desensitization may not work; it also has to be carried out with great caution 'since it could be dangerous if a severe allergic reaction occurs, as happens very rarely. It is usually reserved for people who suffer badly from an allergy and where other treatment has failed.

Antihistamine drugs do not stop an allergic reaction but they do counteract some of the irritating symptoms produced by the chemical histamine. With some of the older types unwanted side-effects such as drowsiness occurred, but the newer antihistamines do not have these effects.

Steroid drugs that partially suppress the immune allergic reaction may also be used to treat allergies. 'Topical' steroids (applied to the skin, as creams) are often used to treat skin reactions such as allergic dermatitis. Sodium cromoglycate (Intal) is another drug sometimes prescribed for hay fever and asthma sufferers, and it is also available as eye drops for allergic conjunctivitis.

Further details of specific allergic disorders are given in the sections dealing with the systems they affect.

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