These killer T-cells—sometimes called CD-8—belong to the lymphocyte group of white cells. They are genetically determined to be specific to the patient from whom they come. Cells from one person can almost never be used to treat another person, with the exception of an identical twin. In addition, these killer cells of the immune system target a single antigen molecule embedded in the surface of a body cell. When they identify and kill the antigen-bearing cell, they protect us from cancer. When they target a virus-infected cell, they destroy the infected cell before the virus it contains can reproduce, thus cutting off the progress of the viral infection.