Histamine is a molecule secreted by certain body cells, predominantly white blood cells, when the body comes into contact with a substance to which it is hypersensitive. It also plays a role in the digestive tract and stimulates the secretion of gastric juices, needed to digest food and neurologically for waking functions. In allergic phenomena, histamine is released by certain white blood cells and its action will generate a dilation of small blood vessels called capillaries, and increase the release of water from the vessels, which is responsible for edema (localized swelling). In addition, this chemical mediator is responsible for the itching present in numerous allergies. To counteract the symptoms following the release of histamine, antihistamines oppose its effects.