Histamine is a chemical that your immune system releases to send messages between cells, according to MedlinePlus.

While histamine is best known for causing an allergic reaction when your body responds to an allergen (a foreign substance), it’s also responsible for many other important functions, per Cleveland Clinic, including:

• Sending messages to the brain
• Telling stomach cells to make stomach acid
• Regulating the sleep-wake cycle
• Helping with cognitive function
• Triggering a response to an injury or infection

Histamine is present throughout the body, but it is mainly stored in mast cells and basophils, according to an article published in StatPearls. Mast cells are white blood cells that are mostly present in loose connective tissue, such as in the lungs, under the skin, in the intestines, near blood vessels, in nerves, and close to lymph vessels. Basophils are another type of white blood cell that form in the bone marrow. When they mature, basophils travel through the bloodstream to damaged tissues in order to heal them after injury.

Normally, histamine works to protect you from foreign pathogens, notes MedlinePlus. But in some people, when the body is exposed to an allergen such as dust, pollen, or dander, the immune system mistakenly thinks the substance is harmful and overreacts. Nemours KidsHealth explains that, in an effort to protect your body, your immune system releases histamine and other chemicals into the bloodstream.

According to Cleveland Clinic, the effect of histamine depends on which receptor it binds to. There are four types of histamine receptors in our bodies:

• H1 receptors are present throughout the body, including in brain cells, smooth muscle cells of airways, and blood vessels. When these receptors activate, they can cause allergy symptoms, such as a runny nose, sneezing, and congestion. Additionally, they can trigger anaphylaxis symptoms, such as itchy skin, low blood pressure, expanding blood vessels, constricted airways, flushing, and increased heart rate. H1 receptors also regulate sleep-wake cycles, body temperature, emotions, memory, and food intake.
• H2 receptors are primarily found in the stomach, smooth muscle cells, and heart cells. When activated, they can cause stomach acid secretion, low blood pressure, flushing, headache, increased heart rate, tightening of airways, and stimulation of mucous glands in airways.
• H3 receptors are found in nerve cells. They regulate the release of histamine and other chemicals, such as dopamine, norepinephrine, and acetylcholine.
• H4 receptors are present in the bone marrow and hematopoietic cells (immature cells that can develop into different types of blood cells). Researchers believe H4 receptors are a factor in inflammatory conditions and autoimmune diseases.

According to StatPearls, some allergic conditions that are related to the release of histamine include:

• Atopic dermatitis (eczema), an inflammatory skin condition that causes dry skin, rashes, scaly patches, and itchiness
• Contact dermatitis, a skin reaction that occurs when someone comes in contact with an irritant, such as fragrances, cosmetics, metals, or plants
• Allergic conjunctivitis, in which eyes become swollen, itchy, red, or inflamed because of a reaction to an allergen
• Allergic rhinitis (hay fever), an allergic reaction that can cause sneezing, a runny nose, congestion, itchy eyes, and a sore throat.
• Allergic asthma, a condition that occurs when allergens trigger asthma symptoms.