Mucosal tolerance is an immunological phenomenon specific to mucosal surfaces as found in the lungs and gastro-intestinal tract. It results in the suppression of immune responses to inhaled or ingested antigens and prevents the body from unwan-ted and unnecessary immuno-logical responses to harmless molecules, such as grass-pollen or food constituents. This imposes the difficult task for the immune system of keeping a balance between reacting and non-reacting, and disturbances of this balance result in allergies and possibly autoimmunity, as well as opportunistic infections and even an escape from tumor surveillance. Understanding the mechanisms that underlie mucosal tolerance is, therefore, important from different viewpoints. Maintenance or (re)induction of mu-cosal tolerance to, e.g., food proteins, airborne allergens or autoantigens is desirable to prevent or cure allergies and autoimmune diseases. However, induction of mucosal tolerance is an unwanted phenomenon in mucosal vaccination and in the case of mucosal tumors.