Maturing T cells in the human immune system need vitamin D to mobilize their defenses, scientists at the University of Copenhagen have found (Nat. Immunol., DOI: 10.1038/ni.1851). The discovery might help researchers determine how to regulate immune response and guide the fight against infectious diseases and autoimmune disorders such as multiple sclerosis. Although vitamin D, which is synthesized with the help of sunlight, has been implicated in MS and other ailments, scientists know relatively little about its role in the immune system. Carsten Geisler and colleagues observed that when naïve human T cells, which aren’t yet primed to respond to threats, first experience an invader, they begin expressing a receptor for vitamin D. If a T cell obtains enough vitamin D, it then gets a boost in the levels of an essential signaling protein that makes it more responsive to pathogens. In contrast, the Copenhagen team determined that dialysis patients with a chronic deficiency of vitamin D had less responsive T cells. The overall findings could be particularly helpful in optimizing vaccine efficiency, Geisler says.