Allergy is a 'supersensitivity' or a form of exaggerated immunological reactivity to some organic substance in the air, in food or on an inert surface, or to certain industrial chemicals. Allergens are usually harmless in themselves, but the reactions they sometimes excite may cause considerable physical distress ranging from a runny nose, a skin rash or wheezing to, at worst, coma. The allergies include asthma, hayfever, urticaria, eczemas, local and systemic anaphylaxis, sensitization to industrial chemicals, and strong reactivity towards tuberculin. Allergic reactivity can be transferred from one individual to another by means of transfusions of blood serum containing the appropriate antibody.
There are different classes of allergic reactions. Type I is most severe and can lead to anaphylactic shock and death; people can have this reaction to bee stings or shellfish. Type IV is a skin rash caused by latex or chemicals in latex gloves.
All around the world, even in developing countries, allergies are on the increase. A 1992 UK report found a 35% increase of the country's allergy sufferers since 1986.
Some allergies are the result of increased pollution of the environment, including food, air and water.