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.