A chemical that is present in cells (mainly mast cells) throughout the body that is released during an allergic reaction (see allergy). Histamine activates 2 main types of receptors, H and H. H activation is responsible for the swelling and redness that occur in inflammation. It also narrows the airways in the lungs and causes itching. H activation stimulates acid production by the stomach. These effects can be counteracted by antihistamine drugs.


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