Histamine causes the following: Local vasodilation (causing erythema) Increased capillary permeability and edema (producing a wheal) Vasodilation of surrounding arterioles mediated by neuronal reflex mechanisms (causing flare—the redness around a wheal) Stimulation of sensory nerves (causing itching) Smooth muscle contraction in the airways (bronchoconstriction) and in the GI tract (increasing GI motility) Increased nasal, salivary, and bronchial gland secretions When released systemically, histamine is a potent arteriolar dilator and can cause extensive peripheral pooling of blood and hypotension; cerebral vasodilation may be a factor in vascular headache.