An allergy occurs when the body reacts to substances it views as harmful. The substances, such as tree pollen, dust mites, or foods, are called allergens. They are usually harmless, but the immune system of a person with allergies defends the body against these allergens.
Following the body? first exposure to an allergen, white blood cells produce antibodies that prepare the immune system for the next encounter with the same allergen. Antibodies attach themselves to mast cells?pecial cells found in the tissues of the respiratory and digestive system. Subsequent exposure to even a small amount of the allergen triggers the mast cells to release chemicals. The release of one of these chemicals, histamine, causes the common symptoms associated with allergies.
For persistent or bothersome allergies, treatment can include desensitization with allergen vaccines, commonly known as immunotherapy or allergy shots. Allergen vaccines consist of a small amount of the substance that causes the allergy. The patient receives injections of the allergen vaccine at regular intervals. The amount of allergen is gradually increased until a dose is reached that is effective in reducing symptoms. In time, this treatment increases the patient's immunity and tolerance to the allergen so that exposure will not cause an allergic reaction.