Histamine: Friend or Foe? …or Frenemy? From NIH MedlinePlus Magazine Histamine: is it the most annoying chemical in the body? Blah It’s the stuff that allergies are made of. Hay fever? Food allergy? Skin allergies? Histamine plays a big role in all of them. And those conditions play a big role in us. In 2015, CDC data showed that more than 8% of US adults had hay fever. More than 5% of US children had food allergies. And at least 12% of all US kids had skin allergies! So what’s the deal? Why do we have such a pesky chemical in our body? Well, histamine is usually our friend. Histamine is a signaling molecule, sending messages between cells. It tells stomach cells to make stomach acid. And it helps our brain stay awake. You may have seen these effects illustrated by medicines that block histamine. Some antihistamines can make us sleepy and other antihistamines are used to treat acid reflux. Histamine also works with our immune system. It helps protect us from foreign invaders. When the immune system discovers an invader, immune cells called B-cells make IgE antibodies. The IgE’s are like “WANTED” signs that spread throughout the body, telling other immune cells about specific invaders to look for. Eventually mast cells and basophils pick up the IgE’s and become sensitized. When they come in contact with a target invader… They spew histamine and other inflammatory chemicals. Blood vessels become leakier, so that white blood cells and other protective substances can sneak through and fight the invader. Histamine’s actions are great for protecting the body against parasites. But with allergies, the immune system overreacts to harmless substances, not parasites. This is when histamine becomes our foe. Common allergens include peanuts, pollen, and animal dander. Leaky vessels cause tearing in eyes, congestion in the nose, and swelling…basically anywhere. Histamine works with nerves to produce itching. In food allergies it can cause vomiting and diarrhea. And it constricts muscles in the lungs, making it harder to breathe. Most worrisome is when histamine contributes to anaphylaxis, a severe reaction that is potentially fatal. Swollen airways can prevent breathing, and a rapid drop in blood pressure could starve organs of vital blood. So what can be done about histamine? Antihistamines block cells from seeing histamine and can treat common allergies. Medicines like steroids can calm the inflammatory effects of allergies. And anaphylaxis needs to be treated with a shot of epinephrine, which opens up airways, and increases blood pressure. So our relationship with histamine is…complicated. We can do better. NIH and specifically the National Institute of Allergy and Infectious Diseases (NIAID) support research of histamine and its related conditions. Great progress is being made in understanding allergy triggers and managing allergic symptoms, and figuring out why histamine, our frenemy, acts the way it does. Find out specific up-to-date research and stories from medlineplus.gov and NIH MedlinePlus the magazine, medlineplus.gov/magazine, and learn more about NIAID research at niaid.nih.gov.