Histamine is a substance that occurs in almost all vegetable and animal organisms just as it occurs in the human body.

Histamine is a biologically-active organic amine molecule which is found throughout living systems, including plants and animals, and also in humans. Our bodies contain histamine within our cell tissue, specifically in tissue mast cells and in blood cells known as basophils. Histamines play a wide and varied role in many of our bodily functions, especially the immune system and as neurotransmitters.

One of the most important uses of histamine is in immune response and how the body deals with inflammation or infection. When tissue mast cells are damaged, they release histamine molecules; these then cause any nearby blood vessels to dilate. By also increasing the permeability of the blood vessels, leukocytes and blood plasma proteins from the immune system are released to the area surrounding any inflammation or infection, and can then work to fight the injury.

Histamines are also involved in a range of other activities within the body, including the production of gastric acid in the stomach, transmitting chemical messages across cells, and helping to lower blood pressure through blood vessel dilation. The histamines work by binding to specific histamine receptors on cell walls, known as H1, H2, H3 and H4 receptors - for example the H2 receptor is involved with the stomach acid secretion; in medical treatments, the effect of histamines can be blocked by specifically targeting those receptors. The chemical's activity can also help keep you awake, by virtue of H3 receptor antagonism by histamine neurons.

Histamines are also found in common foods, such as beer and wine, many cheeses, mushrooms, processed meats, dried fruits, eggs and milk; indeed a huge range of common foods contain the chemical to some extent, though some release it more readily than others. Histamine can build up as food ages, for example mature cheeses and wines will generally contain far higher levels of the chemical. Some foods which contain low levels of histamines actively release it; these include many citrus fruits, spices, nuts and tomatoes among others.