An IgM antibody, present in the serum of a female patient with aplastic anemia, is described that reacted in a modified complement-dependent cytotoxicity test with a subset of the B cells from HLA-A2-positive, but not HLA-A2-negative males. With the exception of two HLA-A2 positive females, the antibody did not react with other cells from either HLA-A2-positive or HLA-2-negative females. The cells of one of these and from HLA-A2-positive males were able to absorb the antibody from the serum. Cells from other donors were unable to absorb the antibodies. The mononuclear cells of the same patient were cytolytic in cell-mediated lympholysis (CML) for phytohemagglutinin blasts from all HLA-A2-positive males and one of the females reacting with antibody, but not with blasts from HLA-A2-negative males and all other females. Thus, the results obtained with the antibody in the complement-dependent cytotoxicity test showed an almost perfect correlation with cytolysis in CML tests. These results suggest that the IgM antibody may be the first example with major histocompatibility complex restriction. Because the antibody reacted with the cells from two female donors, the restricting determinant is not, in all probability, the H-Y determinant.

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