Histamine
A chemical mediator released from mast cells and basophils during allergic reactions, causing vasodilation, increased vascular permeability, itching, and smooth muscle contraction. Histamine release contributes to injection site reactions and allergic responses to peptide drugs.
Technical Context
Histamine (2-(imidazol-4-yl)ethylamine) is synthesised from histidine by histidine decarboxylase and stored in mast cell and basophil granules. Four histamine receptors (all GPCRs): H1R (Gαq — smooth muscle contraction, vascular permeability, pruritis, wakefulness), H2R (Gαs — gastric acid secretion, cardiac chronotropy), H3R (Gαi — presynaptic autoreceptor in CNS, regulating histamine/other neurotransmitter release), H4R (Gαi — immune cell chemotaxis, particularly eosinophils and mast cells). In peptide drug adverse reactions: H1R activation causes urticaria, angioedema, bronchospasm, and pruritus; H2R activation contributes to hypotension and tachycardia. Antihistamines (H1R blockers: cetirizine, loratadine; H2R blockers: ranitidine, famotidine) are used as premedication for infusion reactions and as treatment for mild allergic reactions to peptide drugs.