An allergy is a state of abnormal sensitivity in susceptible individuals to one or more foreign materials, called allergens, not normally considered harmful. Allergy is essentially the inappropriate reaction of antibody and antigen defenses to environmental substances. Common allergens include pollen, animal hair, fungi, house dust mites, insect stings, and occasionally foods and some drugs, such as penicillin and aspirin.
Exposure to allergen induces the formation of antibodies; when, at a later date, the material is again encountered, it reacts with the antibodies causing release of histamine from mast cells in the tissues. inflammation follows, with local irritation, redness, and swelling, which in skin appear as eczema or urticaria. In the nose and eyes hay fever results, and in the gastrointestinal tract diarrhea may occur. In the lungs a specific effect leads to spasm of bronchi, which gives rise to the wheeze and breathlessness of asthma. In most cases, the route of entry determines the site of the response; but skin rashes may occur regardless of route and asthma may follow eating allergenic material. If the allergen is injected, anaphylaxis may occur. Localized allergic reactions in skin following chronic exposure to chemicals (e.g., nickel, poison ivy) are the basis of contact dermatitis.
A tendency to allergic reaction is often hereditary but manifestations vary with age. Treatment usually requires the identification of the allergen and its avoidance if possible, or a course of desensitization. Drugs that may be useful in relieving acute symptoms include antihistamines, cromoglycate, and steroids. Adrenaline may be life-saving in severe allergic reactions.