You and your health care professional should consider allergy shots if you have:
Allergy shots are believed to work by adjusting the way that your immune system responds after exposure to your triggers. Several changes occur after several months of allergy shots. Allergy shots gradually stimulate the production of antibodies that do not cause allergy, called IgG (immunoglobulin G) antibodies. The shots gradually lessen the production of “allergy” antibodies called IgE (immunoglobulin E). Your immune system also activates immune cells called “suppressor T cells” once you respond to allergy shots.
Once these changes have occurred, your allergy triggers are less likely to result in a release of histamine, and less likely to cause allergy symptoms. When they work, allergy shots can limit or even eliminate the need for medications to control allergy symptoms.
The downside of this treatment is that allergy shots have to be given regularly for a few years, and even then they don't work for everyone. It takes several months to a year before symptoms begin to improve. In the beginning, shots containing very small doses of an allergen are given on a weekly basis or more frequently. Later, after your immune system has time to begin its adjustment, somewhat larger doses are given and these can be spaced farther apart in time. Eventually you are given a maintenance dose about once a month.
It is necessary to remain in your physician’s office for at least 20 to 30 minutes after getting allergy shots to make sure you don't have a reaction. Sometimes, reactions are as simple as hives or irritation at the injection site. Reactions requiring immediate medical attention, such as an asthma attack or anaphylaxis, are rare but can be life threatening. This period of observation allows you to get treatment at the earliest signs of a reaction, if one occurs.