Figure 1.

TLR5 is in the intestine and liver for the recognition of flagellin and beyond. (a) TLR5 is expressed on the basolateral side of intestinal epithelial cells and functions in the normal surveillance of flagellated bacteria. (b) The liver receives the intestinal venous blood circulation and arterial supply and acts as a functional vascular firewall that clears commensals that have penetrated either the intestinal or systemic vascular circuits. (c) TLR5 is expressed at a low level by hepatocytes, which can be activated by flagellin specifically in the liver. TLR5 signaling in the liver modulates local immunity by secreting cytokines and chemo-attracting immunocytes, such as neutrophils, macrophages, NK cells, dendritic cells and T cells. These cells may help to eliminate bacteria directly and regulate specific immune responses.