- A standard treatment for SAA involves injections of rabbit-antithymocyte globulin (r-ATG). r-ATG is developed by injecting horses with a type of human white blood cells called thymocytes. The horse's immune system reacts against these cells and makes antibodies that can destroy them. These antibodies are collected and purified to make r-ATG. Horses can also be used for this procedure to make horse-antithymocyte globulin (h-ATG).