A large number of studies have consistently shown that growing up on a farm in various rural areas in Europe confers protection from the development of hay fever, atopic sensitization, and less consistently of asthma from childhood into young adulthood. Exposures to livestock as well as consumption of unpasteurized milk are likely to be distinct and relevant sources of protective exposures. In turn, the underlying microbial exposures have not been identified with certainty. Although environmental exposures to bacterial and fungal components have been found to be inversely related to asthma and atopy, they do not explain the "farming effect." The mechanisms conveying the protection are still poorly understood. An important role for innate immune responses is suggested by findings relating to increased expression of genes of Toll-like receptors in exposed children. How this activation of innate immunity is translated into reduced IgE-specific adaptive immune responses remains to be elucidated, but may invoke a number of distinct allergen-specific steps.