A recently developed monoclonal antibody-specific immobilization of platelet antigens (MAIPA) was evaluated as a method to detect platelet specific antibodies. Antisera to Siba and Koza which were responsible for the refractoriness to platelet transfusion were reactive only with immobilized GPIb. They were not reactive to MoAb-immobilized HLA-classI, GPIIb/IIIa or IV. Therefore, the Siba/Koza epitopes appeared to be on GPIb molecule. Besides anti Siba and anti Koza, alloantibodies to GPIIb/IIIa or Ib were detected in the sera of patients who were refractory to HLA-matched platelet transfusion by the MAIPA method. In several cases, the specificities of such antibodies did not correspond to the known platelet antigens.
