Histamine

Histamine is a biogenic amine that is involved in local immune responses as well as regulating physiological function in the gut and acting as a neurotransmitter for the brain, spinal cord, and uterus. Histamine is involved in the inflammatory response and has a central role as a mediator of itching. As part of an immune response to foreign pathogens, histamine is produced by basophils and by mast cells found in nearby connective tissues. Histamine increases the permeability of the capillaries to white blood cells and some proteins, to allow them to engage pathogens in the infected tissues.

Production and regulation

Histamine is derived from the amino acid histidine, which is converted to histamine by an enzyme called histidine decarboxylase. It is stored in granules in mast cells and basophils and is released as part of an immune response. Once released, histamine can cause a variety of responses including vasodilation, increased vascular permeability, and stimulation of sensory nerve endings.

Role in immune response

Histamine plays a central role in the immune response, acting as a mediator in the processes of inflammation and allergy. It is released by mast cells and basophils when these cells are activated by an immune stimulus, such as the presence of an antigen. Once released, histamine binds to specific receptors on various cells, triggering a range of responses that include dilation of blood vessels, increased secretion of gastric acid, and stimulation of the heart.

Histamine receptors

There are four known types of histamine receptors, designated H1 through H4. H1 receptors are found on smooth muscle, endothelial cells, and central nervous system tissue, and mediate contraction of smooth muscle, increase in capillary permeability, and neurotransmission. H2 receptors are located on parietal cells and vascular smooth muscle cells, and mediate gastric acid secretion and vasodilation. H3 receptors are found in the central nervous system and to a lesser extent the peripheral nervous system, where they inhibit the release of histamine and other neurotransmitters. H4 receptors are found primarily in bone marrow and white blood cells, and regulate immune response.

Histamine in disease

Histamine is involved in many types of allergic reactions. These reactions can range from mild, such as hay fever, to severe, such as anaphylactic shock. Histamine is also involved in conditions such as gastritis and gastroenteritis, where it stimulates the production of stomach acid.

See also

• Antihistamine
• Histamine antagonist
• Histamine intolerance
• Mast cell

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