The allergic immune response is almost always established in childhood. Allergies result from an over-reaction to harmless proteins in allergens to which the immune system is not supposed to react and are due to a defective number or function of regulatory T-cells. These T cells can be stimulated by the normal flora in the gut and it is thought that an altered gut flora early in life is a cause of poorer regulation of the immune response to allergens. The project follows children from birth to six years of age to study the relationship between gut microbiota, regulatory T cells and allergic immune response and disease. This is a unique project as it is prospective and a collaboration between immunologists, bacteriologists and pediatric allergists. Preliminary data show an association between a type of bacterium in the gut early in life and increased numbers of regulatory T cells, and an association between increased numbers of regulatory T cells and allergic disease. The aim of the project is to find support for administering a killed bacterium or bacterial product on the intestinal mucosa early in life to prevent the development of allergic disease.