Allergy is the term used to describe an adverse (bad) reaction that the body has to a particular substance. Most things that cause allergies are not harmful and have no effect on people who are not allergic. Allergies are very common and affect around one in four people in the UK at some time in their lives. Any substance that triggers an allergic reaction is called an allergen. There are many different types of allergens, but three of the most common are pollen, dust mites and nuts.