What Is an Allergy?
An allergy is an abnormally high sensitivity reaction to an ordinarily harmless substance. The immune system of an allergic person reacts when an allergen is absorbed into their body, treating the substance as a harmful invader, causing the white blood cells to begin producing IgE antibodies which attach themselves to mast cells. When this occurs it causes the body to release histamine, which can cause allergy symptoms -- watery eyes, itching, sneezing or a runny nose.

National Institute of Allergy and Infectious Diseases