Histamine is a biologically active amine compound with a molecular formula of C5H9N3. It is formed by the decarboxylation of the amino acid histidine and is widely distributed in animal and plant tissues throughout nature. In particular, it is notoriously found in the venoms of many insects, such as wasps and bees, and is responsible for the severe allergies that some individuals have to their stings. In humans, histamine functions as a major mediator of inflammation and takes part in many allergic reactions by dilating blood vessels and increasing the vascular permeability of their walls. Histamine also stimulates hydrochloric acid secretion in the stomach and acts as a neurotransmitter in the regulation of sleep and wakefulness. See also: Allergy; Amine; Inflammation
In humans, most of the histamine found in tissues is stored inside the resident leukocytes (white blood cells) called mast cells. Mast cells act as sentinels to promote the body's immunological defenses against parasites and bacteria, but they also play a major role in allergic disorders and anaphylactic (acute allergic or hypersensitive) reactions. In response to tissue injuries or the presence of unfamiliar protein fragments (antigens), mast cells release massive amounts of histamine. Histamine is also found in the circulating white blood cells called basophils, which have histamine-containing granules. Beneficial as the immunological effects of released histamine can be, the released histamine may cause itching, wheals, bronchoconstriction, and postnasal drip (runny nose) in certain individuals. In extreme situations, life-threatening anaphylactic shock may ensue, in which case the prompt administration of epinephrine (adrenaline), which opposes the activity of histamine, is vital. See also: Anaphylaxis; Antigen; Epinephrine; Immunology; Mast cells
All of the effects of histamine are mediated by the activation of specific histamine receptors, designated H-1, H-2, H-3, and H-4, which are cellular proteins situated throughout the body that bind with histamine. Antihistamine drugs exert their effects by blocking the combination of histamine with these receptors or by halting the release of histamine, thereby preventing the allergic reaction. See also: Antihistamine
Histamine occurs naturally in many foods, but certain foods can also cause the body to release histamine. Normally, humans possess an enzyme (diamine oxidase) that breaks down any ingested histamines. However, persons who have low levels of this enzyme are prone to suffer histamine intolerance with symptoms and reactions that are typical of food allergies, including headaches, rashes, itching, or abdominal pain. Examples of foods that are histamine-rich or that cause the release of histamines in the body include cheeses, chocolate, beer, mushrooms, processed meats, eggs, shellfish, fermented soy products, and yogurt. Individuals who are histamine-intolerant therefore need to restrict sharply the amount of these foods in their diet. See also: Enzyme; Food allergy; Nutrition
