It has long been recognised that one way to reduce the allergic symptoms of allergy sufferers is to desensitise their immune systems by exposing them to much higher doses of allergen than they are used to - taking care to monitor symptoms under medically controlled conditions as the allergen dose is slowly increased. This often relieves symptoms for several years but will eventually need to be re-done. This is effective for the treatment of hay fever (seasonal rhinitis) and a number of other allergies - there has even been experimentation with severe allergy.
Scientists are now attemptiong to discover why the treatment does not last longer than it does. With standard therapy requiring 3 years of exposure to the allergen it is costly and inconvenient. In this study they tried giving an antigen for only 2 years and found no benefit after 1 year of stopping treatment, whereas treatment for 3 years was effective for 6 years or more. Cost saving is not supported in this instance and treatment is best continued for the full 3 years.