Allergies, also known as allergic diseases, are a number of conditions caused by hypersensitivity of the immune system to something in the environment that normally causes little problem. An allergy refers to an exaggerated reaction by our immune system in response to exposure to certain foreign substances. The response is exaggerated because these foreign substances are usually seen by the body as harmless and non-allergic individuals do not produce a response. In allergic individuals, the body recognizes the foreign substance, and one arm of the immune system generates a response. Allergy-producing substances are called "allergens."