Transplantation tolerance.

New experimental protocols for the induction of transplantation tolerance continue to be developed. In the past year, encouraging data have been reported from a clinical trial using a protocol specifically designed to induce tolerance to the histocompatibility antigens of the kidney donor. Progress has also been made in our understanding of the mechanisms responsible for the induction and maintenance of tolerance to alloantigens in vivo; it is becoming increasingly clear that more than one mechanism can be involved, particularly at different phases in the response.