Abstract.A proportion of Melanesians possess an outstandingly weak ITantigen. Their red cells are ITnegative in the sense that cord bloods are I negative. Of Papuan coastal dwellers, 15.4% of 532 subjects tested were ITweak. This proportion is not constant among Melanesian ethnic groups. Of 419 New Guinea Highlanders, none was ITweak. Similarly, of 171 Caucasians tested, all had normal ITcontent. The phenomenon can be demonstrated on Melanesian cord bloods, and family studies confirm the genetic basis, ITw presumably reflecting a recessive homozygous state.The place of ITin the Ii system remains obscure, but the demonstration of genetic variation in this antigen independent of I and i strength, and occurring frequently in certain ethnic groups, clearly reveals hitherto unsuspected complexity.
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