Allergies are an exaggerated immune system response to a substance the body perceives as a threat. If the trigger substance is introduced in regular small doses, the immune system will become less sensitive to it, and your body will build up a tolerance. This should translate to a reduction in symptoms.
First, the allergen responsible for your symptoms will need to be identified through an allergy skin test. Then a liquid extract of the substance is prepared for daily use. The drops should be held under your tongue for a minute or two, and then swallowed. While effective in the long run, it takes a good three to five years for the majority of patients to develop a tolerance to the allergen.