Scientists believe that aplastic anemia happens when the immune system attacks and destroys bone marrow stem cells. ATG kills the specific cells that are attacking the bone marrow stem cells. This allows the bone marrow to grow and make new blood cells. ATG may be used to treat other bone marrow failure diseases in some cases. ATG is approved by the U.S. Food and Drug Administration (FDA) for treating moderate and severe aplastic anemia. ATG is commonly used with another drug called cyclosporine.