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Hay fever. Computer illustration of a person inhaling pollen grains. Pollen grains are released when flowering plants reproduce. 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 inflamed, 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. (KEYSTONE/SCIENCE PHOTO LIBRARY/KATERYNA KON/SCIENCE PHOTO LIBRARY)