A universal donor is someone who can donate blood to anyone else, with a few rare exceptions. People with the O- blood type have traditionally been considered universal blood cell donors. Conversely, a universal recipient can safely take blood from anyone, again with a few exceptions. People with AB+ blood have been considered universal recipients. Historically, universal donors are determined on the basis of the ABO blood typing system. Under this system, people can be divided into four blood types: A, B, AB, and O. Blood type is determined by the antigens present on the blood cells.