ABSTRACT
Transplantation tolerance is traditionally defined as donor-specific unresponsiveness (3). Implicit in this definition is that tolerant recipients are unresponsive to donor antigens, but maintain reactivity to other (third-party) antigens. For example, a tolerant patient is someone who is capable of mounting an effective immune response against vaccines or pathogens, but is incapable of rejecting the transplanted organ after withdrawing immunosuppression. Donor-specific unresponsiveness, however, is rarely observed in the clinic, and patients with long-term functioning allografts off immunosuppressive medications often retain normal immunological reactivity against their donors. These observations have led to alternate definitions of transplantation tolerance such as “allograft acceptance in recipients on minimal or no immunosuppression,” and have spurred terminologies such as “operational” and prope tolerance.
