The mammalian alimentary tract harbors hundreds of bacterial species that constitute the indigenous microbial flora. The indigenous microbial flora has long been appreciated for its role in host immune system development. Recent reports suggest that components of the microbial flora differentially affect the proportion and number of functionally distinct subsets of T cells in the intestine. Substantial changes in the composition of the microbiota are associated with inflammatory bowel disease. This review will discuss the importance of individual species of microbial flora in the induction of T cell subsets, particularly Th17 cells and regulatory T (Treg) cells in the intestine.
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