Few things are more tragic than desperate transplant patients who finally receive a long-awaited organ only to have their bodies reject it. Now there may be a way to prevent that. In a study of five kidney-transplant patients whose donors were not exact matches, doctors primed the recipients' immune systems by lowering their level of protective T cells, then gave them some of the donor's bone marrow at the time of surgery. This created a sort of hybrid immune system. Four of the patients accepted the kidneys and were able to go off immunosuppressive drugs within a year of surgery.