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Health A-Z

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Harvard Medical School

What It's Used For

Common allergic symptoms are sneezing, runny nose, itchy eyes or asthma attacks. Allergy 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.

Allergy shots, also known as allergen immunotherapy, cause the body to stop generating symptoms after exposure to certain allergens. Tiny amounts of the offending substance are injected under the skin with each shot. The shots stimulate the immune system just a little each time. Gradually, over weeks and months, the amount of allergen is increased.

This very constant, low-level exposure stimulates a different type of immune reaction against the allergen. This new pattern of immune reaction substitutes for and is less bothersome than a traditional allergic response.

Allergy shots can be a good long-term solution when they work well. For people who respond to the treatment, allergy shots can make allergy symptoms less severe and can cause them to occur less often.

Many people benefit from allergy shots for many years after going through a full course of shots. A full course is three to five years. It can take about six months to a year for symptoms to start to subside. For some people, there may be no or little effect even after a year of treatment--in this case, it is not worth continuing the treatment.

Allergy shots are recommended for people with severe allergy symptoms who do not respond to usual medications. They are useful for people who have significant side effects from their medications. They can also be useful for people who find their lives disrupted by allergies, or people for whom allergies might become life threatening, such as people who develop asthma attacks or a severe allergic reaction called anaphylaxis. Allergy shots can be used to reduce the severity of reactions to insect stings.

Not all allergies can be treated with allergy shots. Food allergies are not treated with allergy shots. The risk for having an anaphylaxis reaction is too high. However, some children with food allergies have successfully been treated with a treatment that is similar to allergy shots—oral immunotherapy. Oral immunotherapy is given by mouth, not as a shot. It is still considered experimental, and is not yet widely available. Avoidance of foods to which you are allergic currently the best strategy for preventing food allergy reactions.

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From Health A-Z, Harvard Health Publications. Copyright 2007 by the President and Fellows of Harvard College. All rights reserved. Written permission is required to reproduce, in any manner, in whole or in part, the material contained herein. To make a reprint request, contact Harvard Health Publications. Used with permission of StayWell.

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