We need to look at histamine first.
Histamine is one of the chemicals released by your body when it thinks it’s under attack.
When you come into contact with something your body perceives as a threat it sends a signal to mast cells lining your skin, respiratory tract (both upper and lower) and gut. These mast cells then release histamine.
Histamine is as part of an inflammatory response in which various fluids and white blood cells come to the site of the attack to help. Histamine also gets the body to produce more mucus to flush out anything that doesn’t belong. In addition, it causes itching to make you scratch harmful material off your skin. All these reactions are designed to expel any intruder from your body as fast as possible.
But in allergy the “intruder” is a substance that is normally utterly harmless. Your over-eager immune system mistakenly identifies pollen, house dust mites, pet dander – whatever the trigger is – as a threat. It sends out a gush of histamine and you get allergy symptoms such as a rash, sneezing, coughing, runny nose and watery or itchy eyes.