A hormone/chemical transmitter (biogenic monoamine , similar to serotonine , epinephrine , norepinephrine ) involved in local immune responses, regulating stomach acid production and in allergic reactions as a mediator of Immediate Hypersensitivity .

When released from mast cells , histamine causes vasodilation and an increase in permeability of blood vessel walls. These effects, in turn cause the familiar symptoms of allergy including a runny nose and watering eyes. When released in the lungs, histamine causes the airways to swell shut in an attempt to close the door on offending allergens and keep them out. Unfortunately, the ultimate result of this response is the wheezing and difficulty in breathing seen in people with asthma - an occasionally deadly allergic complication which kills an estimated 4000 Americans yearly. See also : Antihistamine .