https://www.scientificamerican.com/article/could-newly-found-peacekeeping-cells-be-a-weapon-against-covid-19/

To fight a respiratory infection, the body needs a two-pronged attack. First, it sends immune cells to the scene to destroy the pathogen. Then the defense system must keep those first responders from spiraling out of control. If this attempt at “peacekeeping” fails, a run-of-the-mill fever and cough can escalate to a life-threatening illness—which happened to the tens of thousands of COVID-19 patients who have succumbed to the global pandemic caused by the SARS-CoV-2 virus.

For the most part, macrophages—the large immune cells that consume pathogens—are first responders. In the lungs of mice infected with viral influenza, however, a small subset of these white blood cells does just the opposite: They suppress excess inflammation, researchers report today in the journal Science Immunology. These peacekeeping macrophages also reside in human lungs, suggesting they “might be very important to help COVID-19 patients resist inflammation and maybe survive,” says immunologist Yufang Shi of the First Affiliated Hospital of Soochow University in China. The hospital sent staff and supplies to the nation’s city of Wuhan, the likely origin of the pandemic, but Shi was not involved in the new study.