Several lines of evidence show that adequate mi-crobial exposure in terms of quantity and diversity is crucial for the development of tolerance and maintenance of mucosal epithelial integrity and homeostasis. Urbanization, sedentary lifestyle, and modern amenities and cleaning practices have, however, markedly reduced exposure to environmental microorganisms, which is considered to be 1 of the major reasons for the current asthma and atopy epidemics in the western world.
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