Symptoms of allergy are caused primarily by the action of histamine, a chemical released from mast cells, which are specialised cells found in most tissues and are involved in inflammatory and allergic reactions. Antihistamines bind to histamine receptors in small blood vessels, as well as in the nasal passages, eyes, skin or any other tissue involved in an allergic reaction, blocking the action of histamine. This relieves symptoms like nasal congestion runny itchy nose, sneezing, tearing, itchy, red, and puffy eyes. Also red itchy lumps on the skin (hives) and other symptoms of allergy.
Most antihistamines are non-sedating because they only act locally; they include fexofenadine, loratadine and cetirizine. Promethazine is a sedating antihistamine because as well as blocking histamine receptors in the tissues, it also passes into the central nervous system and binds to other histamine receptors in the brain, which are involved in stress and anxiety reactions, blocking these responses.
Antihistamine eye drops
Olopatadine is an antihistamine that is used in eye drops to treat symptoms of allergic conjunctivitis, which is irritation and inflammation of the eye due to contact with an allergen like pollen. Olopatadine blocks histamine action by binding to histamine receptors on the conjunctiva (membrane covering the white part of the eye or sclera) of the eye as well as blocking histamine release from mast cells in the eye.
Antihistamine nasal spray
Azelastaline is a fast-acting, long-lasting antihistamine used as a nasal spray to relieve symptoms of allergy. It works by binding to histamine receptors in the cells of the nasal passages and blocks the action of histamine; it also acts directly on mast cells to reduce the amount of histamine released.