Allergic reactions begin in your immune system. When a harmless substance such as dust, mold, or pollen is encountered by a person who is allergic to that substance, the immune system may over react by producing antibodies that "attack" the allergen. The can cause wheezing, itching, runny nose, watery or itchy eyes, and other symptoms.

Antihistamines and nasal sprays handle the symptoms, but they don't try to correct your immune response, which is the root cause of the symptoms. Allergy immunotherapy trains your immune system to not attack the harmless allergens and not trigger the allergy symptoms.