Researchers have identified a new member of the important B7 family of immune system “co-stimulators.” Co-stimulators are molecules that are capable of turning the immune system on or off — and in the process, profoundly affecting human health. Mayo Clinic researchers named this newest molecule B7-H4. It inhibits the action of T cells, the immune system warriors whose basic job is to attack invaders. Turning off T cells helps transplant patients accept foreign organs. But turning off T cells harms cancer patients — their tumors continue to grow without defensive attacks by T cells. The Mayo Clinic report appears in the June 17 issue of Immunity.