The protein subunit of bacterial flagella. Flagellin makes up the filament.
The protein is well-conserved throughout Eubacteria, as all eubacteria that use flagellin to move have a flagellin gene. Though portions of the gene that are exposed on the surface vary wildly among species and subtypes, the portions that make up the inner core that interact with other flagellin molecules and other molecules that make up the flagellum is well-conserved.
The surface portion of flagella is frequently a target of recognition by the immune system of higher organisms. It was long known that antibodies that recognize bacteria frequently recognize the flagella of bacteria. This fact fit in nicely with the fact that certain bacteria possess two separate flagellin genes. By altering which flagellin gene they express, they are able to evade the immune system.
The reason for flagellin's immunogenicity was explained when I discovered and published the fact that flagellin is specifically recognized by TLR 5 (Toll-like Receptor 5), a component of the immune system1. This interaction activates the immune system, notifying the system that a flagellated bacterium is present. In effect, this recognition of flagellin by an innate immune receptor allows it to act as its own adjuvant.
, et al. Nature
. 2001 Apr 26;410(6832):1099-103.