Allergy means "altered working." Watery eyes and a runny nose are great when your body is fighting off a virus to help wash away the offending germs. But when there's no actual threat to the body, these uncomfortable symptoms serve no purpose.
Immune cells, called IgE, are created by the body in response to specific allergens. Once created, they can last for months, immediately releasing histamine and causing symptoms whenever the offending substance is eaten or breathed in. Anti-histamines suppress allergy symptoms, because histamine is the chemical responsible for them.
Environmental allergies can be caused by pollen, animal dander, smoke, or chemicals such as air fresheners or pollution. Pollen is the egg-shaped male reproductive cell of flowering plants. It is typically smaller than the width of a human hair and often carried by the wind from one plant to another.