The expression of HLA Ag on polyploid early human embryonic stages, obtained in an in vitro fertilization and embryo transfer program, was investigated by indirect immunofluorescence tests using a panel of mAb. Neither HLA class I Ag, beta 2-microglobulin, nor HLA class II molecules could be detected on blastomers. The zona pellucida also lacked these Ag, but granulosa cells expressed HLA class I Ag, beta 2-microglobulin, and HLA class II Ag. These results make it likely that the absence of HLA Ag is one of the mechanisms involved in protecting the implanting embryo from rejection by the immunocompetent mother.

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