An allergy is an excessive reaction to a substance that in most people causes no reaction.
Significant allergies occur in 10% of the population.
An allergy may be triggered by almost any substance including foods, pollens, dusts, plants, animals, feathers, furs, mould, drugs, natural or artificial chemicals, insect bites and gases.
Some individuals are far more susceptible to a wide range of substances than others and the tendency to develop allergies may be inherited.
Allergy reactions may be very localised (eg. at the site of an insect bite, or in just one eye), may occur suddenly or gradually, may last for a few minutes or a few months, may involve internal organs (eg. lungs), or be limited to the body surface (eg. skin or nose lining).
When a person is exposed to a substance to which they are allergic, the body reacts by releasing excessive amounts of histamine from mast cells that are found in the lining of every body cavity and in the skin. Histamine is required at times to fight invading substances, but when released in excess, it causes tissue inflammation and the allergic reaction.