T-cell responses against foreign antigens/allergens are common findings very early in infancy and these responses are generally Th2-skewed. Normally, these responses deviate into low level Th1-type of immunity over the first years of life. In atopic children, however, the Th2-skewing persist, resulting in persistent IgE responses against allergens and the development of allergic diseases. In healthy children, tolerance induction by environmental and immunological factors seems to be related to an immune deviation towards a Th1-type of immune response. Hypothetically, immune deviation could be achieved in atopic children by vaccination with inhalant allergen and will conceivably prevent sensitization and development of respiratory allergy. This hypothesis is currently being tested in Sweden and Germany.