When an allergen can not be avoided, allergen immunotherapy is often the only viable solution. Here, tiny amounts of the allergen are injected under the skin in gradually increased doses until a maintenence level is reached. This stimulates the body to block or neutralise certain antibodies (cf. IgE ) that are produced in response to the allergen and are thus responsible for the allergic symptoms experienced.

Multiple theories have been advanced to explain the mechanism of how allergen immunotherapy works. Most researchers agree that three major events commonly occur in patients who receive a course of allergen immunotherapy :

1. The production and release of many of the proinflammatory mediators (particularly cytokines ) are diminished. This may be via a direct effect on mast cells and eosinophils or an immunoregulatory effect mediated by specific populations of lymphocytes .

2. It is common to find increasing amounts of allergen-specific IgG circulating in the plasma of patients receiving allergen immunotherapy. Such IgG could also bind to the specific allergen and prevent its interaction with mast cell-bound IgE.

3. Finally, it can be demonstrated that, after an initial rise, allergen-specific IgE levels in the plasma fall with allergen immunotherapy. This is thought to be due to active immunoregulatory mechanisms that alter how a specific individual responds to a particular allergen.

Not all mechanisms are likely to be active in every treated patient. Also, this form of treatment varies in efficacy among different types of allergy and between individuals. Dust , pollen , mite , dander and insect venom allergic reactions usually respond best. Researchers are trying to determine exactly which mechanisms are active in a specific patient so allergen immunotherapy can be better tailored to the individual. Also, work is ongoing to better chemically define the treating allergens, make allergen immunotherapy safer and safely increase the interval between injections.