Mixed Leukocyte Culture Response in HL‐A “Identical Individuals”

Eight sets of unrelated HL‐A identical individuals and five pairs of parent‐child HL‐A identical persons were tested for response in oneway MLC. Among 52 unrelated HL‐A identical combinations, approximately 65 per cent of the one‐way combinations and 90 per cent of the reciprocal combinations resulted in significant stimulation. Simultaneous serotyping with 200 highly selected and unclassified antisera revealed that these individuals were not actually HL‐A identical. Two types of discrepant reactions were observed: (i) attributable to new antigens possibly of a third HL‐A segregant series and (ii) attributable to the ‘variants’ of certain well‐defined HL‐A groups, e.g., HL‐A2. Though the conclusion is that compatible unrelated donors will be more difficult to find than previously estimated, progress toward this goal is indicated by the present findings that a third of the unrelated combinations selected by phenotyping for 27 antigens were nonstimulatory by mixed leukocyte culture.