Hay fever. Artwork depicting hay fever showing pollen grains (yellow/brown) entering the nasal cavity (red, centre) of a hay fever sufferer. Pollen grains are released in their billions during plants' reproduction. Some people become sensitised to pollen in the air, and have an allergic reaction (rhinitis, or pollinosis) whenever they are in contact with it. The symptoms of an attack include red streaming eyes and a running, blocked nose. The symptoms are caused by a massive release of the chemical histamine in the body in response to the pollen. Drugs which block the action of histamine are of some use in controlling the symptoms.