A substance found in basophil and mast cells that is released as part of an allergic reaction, causing vasodilation, constriction of smooth muscles such as those found in the respiratory tract, increases capillary permeability and can cause itching, tearing, hives, nausea, diarrhea, anaphylaxis, etc. Antihistamines, Benadryl and epinephrine are given over counter depending on severity of reaction.