immune system is so low. >> listen to this. even though her sister's a perfect match, robin's system will still try to attack these donor cells. doctors will work to prevent any symptoms of graft versus host disease when there is rejection of bone marrow transplant. so even though it's a perfect match, it's still a battle to make sure it's not rejected. >> it's still unclear how long she will be out. it's determined by how her body responds to the bone marrow transplant. the markers typically come at 30 days and then 100 days post-transplant. just keep her in your thoughts and prayers. all going to be how her body reacts. >> there is nobody stronger than that woman, right? >> i can't think of a single person. >> incredibly inspiring. >> it's why i'm wearing my little bracelet. >> no doubt about it. tim tebow answers to a higher power. is higher office in his future? >> that's very interesting. what's next after the gridiron glory is gone? you are watching "world news now." >> announcer: abc's "world news now" will continue after this