Histamine is a vasoactive chemical.
Histamine affects nearby nerves resulting in itching. Histamine is stored in granules within mast cells together with other vasoactive chemicals. H1 blockers Antihistamines are available for oral use as tablets and elixirs. They take about 15 to 30 minutes to be effective. Antihistamines are valuable drugs to reduce symptoms due to allergic diseases, especially those mediated by histamine.