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Allergy is the term used to describe an adverse (bad) reaction by the body to a particular substance. Most things that cause allergies are not obviously harmful, and have no affect on people who are not allergic.
Any substance that triggers an allergic reaction is called an allergen. There are many different types of allergens; three of the most common are pollen, dust mites and nuts. Allergens contain protein, which is found in all living organisms, and it is the protein that causes the reaction. Some drugs such as penicillin can also cause allergic reactions. They do not contain protein, but they can cause a reaction if they bind to proteins in the body.
An allergic reaction to the allergen happens when you come into contact with it. Contact may be with your skin, or with the lining of your lungs, mouth, gullet, stomach or intestine. If your body reacts badly to a particular substance, you are said to be allergic to it.
Allergic reactions can cause a range of symptoms. Some can be quite mild, and some are serious, very occasionally resulting in death.
Allergies are very common and affect around one in four people in the UK at some time in their lives. Each year the numbers increase by 5%, with many more children being affected.