Histamine Receptors (H1 - H4)

Histamine itself does nothing on its own until it has somewhere to bind. This role is played by receptors, which can be pictured as locks into which histamine fits like a key. There are four types in the human body, and their distribution explains why the exact same histamine can cause hives in one person, a migraine in another, and diarrhea in a third.

H1 receptors are in the walls of blood vessels, bronchi, and intestines, in nerve endings, and in the skin. They are responsible for itching, redness, hives, swelling, a runny nose, constricted bronchi, a drop in blood pressure, and headaches. Standard over-the-counter antihistamines target these very receptors.

H2 receptors are found mainly in the gastric mucosa, where they trigger the production of stomach acid, and in the heart, where they speed up the heart rate. They explain heartburn, stomach pressure, and heart palpitations after meals.

H3 receptors sit in the nervous system and act as a brake. They regulate alertness, sleep, appetite, and the ability to concentrate. Thus, they are related to what people with HIT describe as brain fog or insomnia after a late dinner.

H4 receptors are on immune system cells and control their movement to the site of inflammation. They are involved in itching and long-term inflammatory issues.

Medications currently exist only for H1 and H2. Treatments targeting H3 and H4 are still in the research phase.