Histamine plays a crucial role in the body's immune response, acting as a signaling molecule in allergic reactions and inflammation. It is released by mast cells and basophils in response to injury or allergens, causing blood vessels to dilate and become more permeable, which helps white blood cells reach affected areas. This process leads to symptoms like swelling, redness, and itching, commonly seen in allergic reactions. In the medical field, histamine is used in diagnostic tests to assess gastric acid secretion, helping diagnose conditions like achlorhydria or Zollinger-Ellison syndrome. It is also a target for antihistamine drugs, which block histamine receptors to alleviate allergy symptoms, such as sneezing, runny nose, and hives. Additionally, histamine is involved in regulating stomach acid production, making it relevant in the treatment of gastrointestinal disorders. It also influences neurotransmission in the brain, affecting wakefulness and appetite, and is studied for its role in neurological conditions.