Acceptable HLA mismatches for highly immunized patients.

Highly sensitized patients have developed antibodies against many different HLA antigens due to previous pregnancies, blood transfusions or failed transplants. These antibodies cause a positive crossmatch with almost all potential organ donors. As a positive crossmatch is a contra-indication for transplantation, highly sensitized patients have a low chance of transplantation unless special strategies are introduced. One such strategy is the acceptable mismatch program, which has led to transplantation of more than 300 of these highly sensitized patients within Eurotransplant. Centers are participating in the program on a voluntary basis. Before a patient can be included in this program, extensive antibody screening is necessary to define those HLA-A and -B antigens towards which the patient has never formed antibodies. Organ donor selection is based on complete compatibility with the patients own HLA antigens in combination with the acceptable mismatches. If such a combination is identified, mandatory exchange takes place. Despite the success of the acceptable mismatch program, about 25% of the patients will never receive a donor offer. These are patients with rare HLA antigens or rare combinations of HLA antigens. In the last few years, this group of patients has had the advantage of two additional programs running within Eurotransplant. In the HIT (highly immunized tray) program, sera of highly sensitized patients are sent to the different centers and crossmatched with all ABO compatible donors. In the case of a negative crossmatch, mandatory exchange takes place. Secondly, these patients can benefit from the extra points they receive for their waiting time, high antibody reactivity and rare HLA type in the standard Eurotransplant allocation system. We conclude that the application of these three strategies will lead to a significantly increased transplantation rate of highly sensitized patients.