Third party mixed-leukocyte culture test: a potential new method of histocompatibility testing.

The disparity index is an expression of histocompatibility difference between two siblings with identical human leukocyte antigen (HL-A) type who are nonreactive in mixed leukocyte culture (MLC) test. This index is derived from a third-party MLC test. The disparity index was found to be correlated with the severity of graft-versus-host reaction in bone marrow transplantation between HL-A-identical siblings. The disparity index may prove to be a useful means of predicting the severity of graft-versus-host reaction and the outcome of bone marrow graft when HL-A- and MLC-matched sibling donors are studied. The third-party MLC test is offered as a new method of histocompatibility testing. It may provide a useful model for the study of immunogenetics relative to so-called weaker histocompatibility determinants in man and experimental animals.