Allergic reactions to drugs, for example, may cause the eyes to itch and swell. The eyelids and the conjunctive are the ones easily affected in an allergic reaction. Inflammation of the conjunctive caused by an allergy is called allergic conjunctivitis. As such, allergic reactions are an important part of the body's immune defense system. When you are allergic to something, your body's defenses react and produce potent chemicals such as histamine.
Histamine is primarily responsible for watery, itchy eyes and many other typical allergic symptoms.
It is unclear why some persons develop allergies. About 10 to 20 percent of the population has allergies. Symptoms can start at any age but children and young adults are more vulnerable to allergies. Allergens are the substances that cause allergic reactions. The most common allergens are found in our environment.