Heterogeneity of HL-A specificities shown by platelet complement-fixation.

Six sets of unrelated healthy individuals, phenotypically “identical” in each set for the four antigens of the two HL—A segregant series, were tested with 153 antiplatelet antisera by microcomplement-fixation. In five of the six sets the results unequivocally show that the individuals were not identical by complement-fixation reactions. It is proposed that sera which manifest nonidentity between individuals within these sets identify either “variants” of the known HL—A antigens or antigens of a third segregant series of the HL—A system or of a non-HL—A system. Accurate and complete matching for HL—A antigens among unrelated individuals will require further definition of specificities.