An allergy is an abnormal reaction of the body's immune system to a substance in the environment that, for most people, causes no immune response. Any substance that causes allergic reactions in some people is called an allergen. Allergens can be normally harmless substances such as pollen, or noxious substances like pesticides.
Allergies are very common, affecting an estimated 50 million Americans. That means one in every six people in the U.S. has some type of allergy.