The gastrointestinal tract represents a major gateway for infection by potential microbial pathogens, and, at the same time, contains hundreds of species of indigenous microbes. Thus, the intestinal immune system should actively react with potentially pathogenic microbes, while simultaneously remaining tolerant to or unaffected by the vast majority of commensal microbes. The recognition, and perhaps the discrimination, of pathogens and commensal microbes is primarily mediated by pathogen-associated molecular pattern (PAMP) receptors, including Toll-like receptors (TLRs) and nucleotide-binding domain (NOD)-like receptor (NLR) family proteins. The regulation of PAMP receptor signaling is critical in the maintenance of immune homeostasis in the intestine, and an aberrant signaling may result in inflammatory bowel diseases.
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