lhistamine

Lhistamine is not a standard chemical name. Histamine itself is a small biogenic amine and, unlike many amino acids, it does not have stable enantiomers. Therefore there is no recognized L- or D-histamine form. If encountered, lhistamine is usually a misspelling or shorthand for histamine in informal or historical texts.

Histamine is formed by the decarboxylation of the amino acid histidine, catalyzed by histidine decarboxylase. It is stored mainly in mast cells and basophils, and is released in response to immune or inflammatory stimuli, tissue injury, or allergen exposure. Upon release, histamine acts as a signaling molecule to coordinate local immune responses and other physiological processes.

Histamine exerts its effects through four G protein–coupled receptors: H1, H2, H3, and H4. H1 receptors mediate allergy symptoms such as itching, vasodilation, and increased vascular permeability. H2 receptors stimulate gastric acid secretion and contribute to mucosal defense. H3 receptors function as presynaptic autoreceptors or heteroreceptors, modulating the release of histamine and other neurotransmitters in the CNS and periphery. H4 receptors are involved in chemotaxis and regulation of immune cell activity. Histamine signaling underlies many physiological and pathophysiological processes, including allergic reactions, gastric function, and neuroimmune communication.

Histamine is metabolized mainly by two pathways: histamine N-methyltransferase (HNMT) and diamine oxidase (DAO). HNMT transfers a methyl group to histamine, while DAO deaminates it. Metabolites are excreted in urine. Clinically, antihistamines block receptor activity to reduce allergic symptoms, and dietary or enzymatic factors influencing histamine breakdown can affect histamine-related disorders such as histamine intolerance.