Histamine is a molecule derived from an essential amino acid, histidine, and it is produced by decarboxylation through enzyme L-histidine decarboxylase.
Histamine has many physiological and patho-physiological functions: It takes part in the regulation of local blood circulation, in capillary permeability, contraction and relaxation of smooth muscles and blood vessels, secretion of hydrochloric acid in stomach, immediate hypersensitivity responses, allergic processes, inflammatory ones as part of the immune response to external pathogens, tissue healing, and its action has also been observed as neurotransmitter in the nervous system.
Histamine is produced by all living beings, and it is a natural constituent of tissues. Therefore, the main histamine source is the human body. Histamine is made from histidine, it is stored in mast cells (immune system cells) and basophils (granulated blood cells) and excreted through the bowel, being degraded when passing through intestinal mucosa by Diamine Oxidase (also known as amoloride-binding protein or histaminase), located in this area. The second source is the derived from food.
Obviously, the same molecule is involved, but with different functions. One of the main functions of endogenous histamine is its intervention in the immune system, in allergic reactions. It is synthesized within mast cells and basophils of connective tissue and mucosa, then, it is deposited in its secretory granules ready to be expelled by exocytosis any time the known allergen contacts the sensitized must cell producing the allergic reaction.
Nevertheless, histamine location within the body is not specific, as there are four types of histamine receptors (H1, H2, H3 and H4) in different areas. Type H1 is located in smooth muscle cells membrane in vessels, bronchi and intestinal tract, in heart conduction tissue, in some secretory cells and in sensory nerve endings. H2 receptors are mainly located in parietal cells membrane in gastric mucosa, in smooth cells of vessels, in myocardial cells and sinus node, in basophils and in mast cells where H2 receptors act as autoreceptors. Despite H3 receptors’ low-density, presence of this type has been detected in several tissues such as lung, stomach, bowel and pancreas tissues. CNS has the three receptors in histaminergic neurons.
The second histamine source (from food), may represent more problems, as its regulation depends on the person. If some alteration in its metabolism occurs and normal histamine concentrations in blood do not remain (50-70 mg/l), free circulation of this amine in high concentrations triggers undesirable effects, such as migraine, fibromyalgia, asthenia, and atopy.