An allergy is the body's hypersensitivity to substances in the environment. An extreme allergic reaction can result in anaphylactic shock, a life-threatening situation in which a person's airway swells shut and blood pressure drops. Scientists believe allergies originated millions of years ago as a way for the human body to rid itself of parasites and invading worms. Without modern parasites to fight, IgE reacts to other foreign substances in the body. IgE triggers immune cells to release a number of chemicals, one of which is histamine.
Histamine produces hives, watery eyes, sneezing, and itching.
The most common symptom of seasonal allergies is allergic rhinitis, otherwise known as hay fever. Symptoms of allergic rhinitis closely mimic those of the common cold. But there are differences. A cold runs its course in 7 to 10 days. Allergic rhinitis can drag on for weeks or months.