H1, or histamine receptor type 1, is a protein found in various tissues throughout the body. It plays a crucial role in the immune response by mediating allergic reactions and regulating physiological functions such as gastric acid secretion. When allergens enter the body, H1 receptors are activated, leading to symptoms like itching, swelling, and inflammation. In addition to its role in allergies, H1 receptors are also involved in the central nervous system, influencing sleep-wake cycles and appetite. Medications known as antihistamines target H1 receptors to alleviate allergy symptoms by blocking their activation, providing relief to those affected by conditions like hay fever and hives.