Health A-Z

Medical Content Created by the Faculty of the
Harvard Medical School

Allergy Shots (Allergen Immunotherapy)

What Is It?

Allergic symptoms -- the familiar sneezing, runny nose, itchy eyes or asthma attacks -- are caused by the body's reaction to a substance (allergen) that is inhaled, touched or eaten. You also can develop a rash from a substance you touch or get stomach upset, hives or difficulty breathing from something you eat. These allergens cause no symptoms in a non-allergic person, but in an allergic person, an immune reaction against the allergen causes symptoms. In allergy, the body responds to the allergen as it would respond to a dangerous invader, such as bacteria or a virus. The immune system recognizes the substance as foreign and activates an army of antibodies to eliminate the invader. The antibodies bind with the allergen and stimulate the release of chemicals, such as histamine, that cause allergy symptoms.

Standard treatment for mild to moderate allergies involves avoiding the allergen that causes the allergic reaction and taking over-the-counter or prescription medications. But when these options are not enough to get rid of symptoms, if allergies are severe or if you have significant side effects from taking your prescription medications, allergy shots may be recommended.