TLR5 is the receptor for flagellin, the protein that forms bacterial flagella. Unlike most other Pathogen-Associated Molecular Patterns (PAMPs), flagellin does not undergo any posttranslational modifications that would distinguish it from cellular proteins. However, flagellin is extremely conserved at its amino- and carboxyl-termini, which presumably explains why it was selected as a ligand for innate immune recognition. TLR5 is expressed on epithelial cells as well as on macrophages and dendritic cells. Expression of TLR5 on intestinal epithelium is polarized such that TLR5 is expressed only on the basolateral side of the cell, as pathogenic but not commensal microbes cross the epithelial barrier. This ensures that innate immune responses are confined to pathogenic but not commensal microbes (Paul 2004; Hayashi et al. 2001; Gewirtz et al. 2001).