Toll-like receptor-5 and the innate immune response to bacterial flagellin.

The innate immune system identifies the presence of infection by detecting structures that are unique to microbes and that are not expressed in the host. The bacterial flagellum (Latin, a whip) confers motility, on a wide range of bacterial species. Vertebrates, plants, and invertebrates all have evolved flagellar recognition systems that are activated by flagellin, the major component of the bacterial flagellar filament. In mammals, flagellin is recognized by Toll-like receptor-5 and activates defense responses both systemically and at epithelial surfaces. Here, we review the role for Toll-like receptor-5 in mediating the mammalian innate immune response to flagellin, and how this provides for defense against infections caused by many different species of flagellated bacteria.