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Perturbations in the gastrointestinal (GI) microbiota composition that occur as a
result of antibiotics and diet in "westernized" countries are strongly associated
with allergies and asthma ("hygiene hypothesis"). The microbiota ("microflora") plays
a crucial role in the development of mucosal tolerance, including the airways. Significant
attention has been focused on the role of the microbiota in GI development, immune
adaptation and initiation of GI inflammatory diseases. This review covers the post-developmental
functions that the microbiota plays in regulating immunological tolerance to allergen
exposure outside the GI tract and proposes the question: is the microbiota a major
regulator of the immune system?