Health A-Z

Medical Content Created by the Faculty of the
Harvard Medical School

Allergy Shots (Allergen Immunotherapy)

What Is It?

Allergic symptoms — these may be the familiar sneezing, runny nose, itchy eyes or asthma attacks that you have from time to time. Allergic symptoms can also be a rash such as hives or itchy blisters, abdominal pain or diarrhea.

Allergic symptoms are caused by your body's reaction to a substance (allergen) that is inhaled, touched or eaten. Allergens cause no symptoms in a non-allergic person, but in an allergic person who is sensitized to that antigen, an immune reaction against the allergen causes symptoms.

In allergy, the body responds to the allergen in the same way it would respond to fight off infection by a parasite. 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 then trigger immune system cells to release chemicals, such as histamine. This release of histamine is what causes most allergy symptoms.

Once you know what triggers your allergy symptoms, the best way to manage mild to moderate allergies is to avoid the allergen that causes the allergic reaction. Taking over-the-counter or prescription medications can also help. 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.