An immediate systemic allergic reaction to a foreign protein or other substance. The reaction results from the release of histamine, serotonin and other vasoactive substances when allergen combines with IgE antibodies on mast cell surfaces. Mast cell mediators affect more than one organ. Subsequent flow of fluid from the circulatory system as a result of mast cell histamine release can have a local manifestation of hives, or local swelling and rednesss or a sudden blood pressure drop and rapid heart rate with possible fatal acute systemic shock. (e.g., peanut, penicillin, egg or Hevea natural rubber latex allergy)