TLR9 and Crohn disease: Among the 13 TLRs discovered, TLR1-TLR9 have been identified as being expressed in human IECs at low levels, however expression is adjusted during intestinal inflammation; e.g., TLR4 is upregulated in the intestinal epithelium during active ulcerative colitis and Crohn’s disease, whereas TLR3 is downregulated in active Crohn’s disease [83,84].