general term for any agent that blocks receptors for the compound histamine; the two receptor sites are referred to as H1 and H2. H1 receptors mediate histamine- associated allergic reactions, dilate blood vessels and are blocked by the classic antihistamines. H2 receptors mediate the secretion of gastric acid. H2 blockers, generally referred to as histamine blockers, include cimetidine, ranitidine, famotidine, and nizatidine.