Histamine is a tissue hormone. It is also widely distributed in the plant kingdom and in bacteria. Histamine is found almost everywhere in the human body, including in the skin, the lungs, the mucous membrane of the gastrointestinal tract and in the hypothalamus, a part of the brain.
It plays a central role in many normal and pathological reactions, especially inflammatory reactions and allergies. It is used by the immune system to defend itself against substances foreign to the body. Thus, it serves as one of the messenger substances in the inflammatory reaction to cause swelling of the tissue.
In the gastrointestinal tract, it plays an important role in the regulation of gastric acid production and motility, the mobility of the intestines. In the brain, it participates in the control of the sleep-wake rhythm and appetite control.
Foods also contain appreciable histamine concentrations, e.g. strawberries, cheese, tuna, tomatoes, yeast, chocolate, red wine and sauerkraut. Histamine is produced by some plants as a defense agent (e.g., by stinging nettles).