You may have noticed that some of the flagella in this chapter were enclosed within the outer membrane of the cell. We call these “sheathed” flagella. It is a common adaptation in pathogenic species, like this Helicobacter hepaticus. Flagella offer pathogens a great advantage in colonizing their hosts; hosts in turn have learned to use them to identify potential invaders. As a result, the innate immune response of many eukaryotes, from plants to insects to humans, has evolved to recognize the telltale and abundant signal of flagellin proteins in the long filament. If your cell aims to take up residence in such a host, it could therefore benefit from cloaking this strongly antigenic feature.
