Histamines

Histamines

Histamines are organic nitrogenous compounds involved in local immune responses, regulating physiological function in the gut, and acting as neurotransmitters. They are part of the body's response to foreign pathogens and play a crucial role in allergic reactions.

Structure and Synthesis

Histamine is a biogenic amine derived from the decarboxylation of the amino acid Histidine. This reaction is catalyzed by the enzyme histidine decarboxylase. Histamine is stored in granules in mast cells and basophils, which are types of white blood cells involved in the immune response.

Function

Histamines are involved in several key physiological processes:

Immune Response

Histamines are released by mast cells and basophils during an immune response to foreign pathogens. They increase the permeability of the capillaries to white blood cells and some proteins, allowing them to engage pathogens in the infected tissues.

Allergic Reactions

In allergic reactions, histamines are released in response to allergens. This release causes symptoms such as itching, swelling, and redness. Antihistamines are drugs that block histamine receptors to alleviate these symptoms.

Gastric Acid Secretion

Histamines stimulate the parietal cells in the stomach lining to secrete gastric acid, which is essential for digestion. This action is mediated by the H2 histamine receptors.

Neurotransmission

In the central nervous system, histamines act as neurotransmitters. They are involved in wakefulness, appetite control, and cognitive functions. Histamine neurons are located in the hypothalamus and project to various parts of the brain.

Histamine Receptors

Histamines exert their effects by binding to histamine receptors, which are G protein-coupled receptors. There are four known types of histamine receptors:

• H1 Receptors: Involved in allergic responses and regulation of sleep-wake cycles.
• H2 Receptors: Primarily involved in the regulation of gastric acid secretion.
• H3 Receptors: Function as autoreceptors in the central nervous system, modulating the release of histamine and other neurotransmitters.
• H4 Receptors: Involved in immune cell chemotaxis and inflammation.

Clinical Significance

Histamines play a significant role in various medical conditions:

• Allergies: Overproduction of histamines can lead to allergic symptoms. Antihistamines are commonly used to treat these symptoms.
• Gastric Ulcers: Excessive histamine activity can lead to increased gastric acid secretion, contributing to the development of ulcers. H2 receptor antagonists are used to reduce acid secretion.
• Anaphylaxis: A severe, life-threatening allergic reaction characterized by massive histamine release, leading to vasodilation, hypotension, and bronchoconstriction.

Also see

• Allergy
• Antihistamine
• Mast cell
• Basophil
• Gastric acid