A group of immune cells that normally protect against inflammation in the gastrointestinal tract may have the opposite effect in multiple sclerosis (MS) and other brain inflammation-related conditions, according to a new study by Weill Cornell Medicine and New York-Presbyterian researchers. The results suggest that countering the activity of these cells could be a new therapeutic approach for such conditions. The researchers, who reported their findings online on December 1, 2021 in Nature, were studying a set of immune cells called group 3 innate lymphoid cells (ILC3s), which help the immune system tolerate beneficial microbes and suppress inflammation in the intestines and other organs throughout the body. They discovered a unique subset of these ILC3s that circulate in the bloodstream and can infiltrate the brain—and, to their surprise, do not quench inflammation, but instead ignite it.