Platelet transfusion therapy for alloimmunized patients: selective mismatching for HLA B12, an antigen with variable expression on platelets.

There can be wide variation in the expression of the HLA B12 antigen and its "splits," B44 and 45, on the platelets and lymphocytes from the same individual. One hundred sixty-two single donor platelet transfusions mismatched only for this antigen group were administered to 54 alloimmunized patients who were refractory to random donor platelets. Satisfactory increments (one-hour post-transfusion corrected-count increment [CCI] greater than 7,500) were seen following 111/162 transfusions (69%). In 31 patients (57%), all transfusions (n = 85) produced CCI greater than 7,500, and 76% of patients received some transfusions that were satisfactory. Of note is that ten patients had excellent increments despite either preformed lymphocytotoxic antibody against the mismatched antigen or positive lymphocytotoxic cross-matches with the donor. In contrast, poor increments were seen in ten recipients under similar circumstances, implying disparities in antigen expression on the platelets and lymphocytes of different donors. There was no obvious pattern of other donor HLA antigens which could be correlated with these differences. The HLA B12 antigen group is relatively common (found in approximately 25% of the population), and these data indicate that selective mismatching for these antigens can be an effective donor-selection strategy to increase the number of donors for alloimmunized recipients.