Histamine, which has long been implicated as a mediator of allergic airway disease, has multiple effects on airways which are mediated by at least three histamine receptors. H1-receptors mediate bronchoconstriction, vasoconstriction and dilatation, microvascular leak and activation of sensory nerves. H2-receptors mediate vasodilatation in some species and mucus secretion. H3-receptors mediate vasodilatation in some species and mucus secretion. H3-receptors modulate cholinergic neurotransmission, the release of neuropeptides from sensory nerves and allergen-induced bronchoconstriction.