The immune system’s white blood cells, which are produced in the bone marrow, mostly help to defend against bacteria and injury, but sometimes they can turn against the body—for example, in cardiovascular disease, their inflammatory aggression can harm arteries and the heart. New research in Nature Immunology that was led by investigators at Massachusetts General Hospital (MGH) provides insights into the pathways that increase or decrease the bone marrow’s output of these cells. The findings may lead to new treatments for conditions that arise when the balance of white blood cell production goes awry.