How helpful gut microbes send signals that they are friends, not foes
The finding in mice may help explain why a body’s immune system doesn’t kill beneficial bugs

MICROBE MYSTERY Segmented filamentous bacteria (green) hook into the gut walls (pink) of many animals, including mice, birds and humans. Why the immune system doesn’t attack these bacteria, as well as other beneficial gut microbes, has been a mystery.
I.I. Ivanov and D.R. Littman/Mucosal immunology 2010