- Histamine: A substance that plays a major role in many allergic reactions, dilating blood vessels and making the vessel walls abnormally permeable. Histamine is part of the body’s natural allergic response to substances such as pollen. Antihistamines work by preventing the release of histamine from certain cells (mast cells) thereby blocking the allergic reaction.
Source: www.medicinenet.com/script/main/art.asp?articlekey=3765