Allergen immunotherapy is the process by which increasing doses of an allergen (allergy-causing substance) are injected subcutaneously (under the skin) over time as a treatment to prevent allergic symptoms that usually occur on exposure to the allergen.
The term allergen immunotherapy is usually used instead of desensitization because the immunologic basis for this form of treatment is currently unknown.
The job of the immune system is to search for, recognize, and destroy germs and other dangerous invaders of the body, known as antigens. It does this by producing antibodies or special molecules to match and counter each antigen.
Since we come into contact with so many substances every day and ingest a wide variety of foods, drinks, and drugs, the immune system does not attack everything foreign to the body. Rather it selectively seeks out only those germs or other invaders that cause infection or that in some other way pose a potential hazard.
Among people who inherit the potential for allergies, the immune system's selectivity breaks down, failing sometimes to distinguish between the benign and the dangerous, the good and the bad. Antibodies attack harmless food, chemicals, pollen, or whatever else they misread and respond to it as an enemy.
The immune system produces at least five kinds of antibodies, but the principal one that participates in allergic reactions is immunoglobulin E, or IgE.
Every individual has different IgE antibodies, and each allergic substance stimulates production of its own specific IgE. An IgE antibody made to respond to ragweed pollen, for example, will react only against ragweed and not oak tree or bluegrass or any other kind of pollen.
When the antibodies encounter the allergen they are programmed against, they immediately signal the basophils or mast cells to unleash histamine or other mediating chemicals into the surrounding tissue.
It is these chemicals - mainly histamine - that cause the familiar allergic reactions. Histamine released in the nose, eyes, and sinuses, for example, stimulates sneezing, a runny nose, and itchy eyes; released in the lungs it causes narrowing and swelling of the lining of the airways and the secretion of thick mucus; in the skin, rashes and hives; and in the digestive system, stomach cramps and diarrhea.
Based on the clinical evaluation, repeated subcutaneous injections of a solution of the responsible allergen(s) are done once or twice a week in increasing doses until a maintenance dose is reached. This maintenance dose is then injected every 2 to 4 weeks, indefinitely.
Reactions to the treatment may include local skin reactions at the injection site and in some cases, anaphylaxis.