A severe allergic reaction that occurs rapidly and causes a life-threatening response involving the whole body. This reaction can lead to difficulty breathing and shock, ultimately leading to death. An allergic reaction 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 may 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.