Histamine is a chemical that your immune system releases to send messages between different cells. It has several functions, but it is mainly known for its role in causing allergic and anaphylactic symptoms. Here are some key points about histamine:

• Histamine is stored in mast cells in tissues and basophils in blood.
• When you come across your allergy trigger, your immune system sends a chemical signal to "mast cells" in your skin, lungs, nose, mouth, gut, and blood. The message is, "Release histamines," which are stored in the mast cells.
• When histamines leave the mast cells, they boost blood flow in the area of your body the allergen affected. This causes inflammation, which lets other chemicals from your immune system step in to do repair work.
• Histamines then dock at special places called "receptors" in your body. The result is allergy symptoms such as sneezing, tearing up, itching, runny or stuffy nose, coughing, and swelling.
• Histamine also has other important functions, such as regulating your sleep-wake cycle and cognitive function.
• Antihistamines are a common medication that can manage histamine levels.

Histamine can stimulate inflammation, and it is a proinflammatory factor. However, histamine also has a pleiotropic effect that is dependent on its interaction with its four histamine receptors. The pleiotropic actions of histamine due to the different natures of its receptors allow this simple molecule to exert broad and opposing effects on the immune system, highlighting the importance of fine-tune control that promotes a homeostatic environment in the body, balancing important inflammatory reactions to host protection as well as immunomodulation.