An allergy is an abnormal response of the immune system of the body to some type of foreign substance. The substance that triggers the allergic reaction is called an allergen. Initial contact with the allergen does not produce any symptoms. However, with time the immune system begins to form antibodies to the allergen. Future interactions with the allergen trigger more dramatic responses. The response to these sensitizing allergens varies from mild sneezing to respiratory difficulty, shock, and death.
Allergies, also known as allergic diseases, are a number of conditions caused by hypersensitivity of the immune system to something in the environment that usually causes little or no problem in most people. These diseases include hay fever, food allergies, atopic dermatitis, allergic asthma, and anaphylaxis. Symptoms may include red eyes, an itchy rash, runny nose, shortness of breath, or swelling. Food intolerances and food poisoning are separate conditions.