Is restricted antigen presentation the explanation for fetal allograft survival?

Mammalian embryos express paternal histocompatibility antigens which make them potential targets for the maternal immune system. The trophoblast is a major histocompatibility complex (MHC) antigen-negative barrier between mother and fetus which facilitates movement of antigenic molecules but prevents traffic of antigenic cells and is itself unable to present antigen. Gary Wood suggests that the lack of antigen presentation requirements for MHC class I-restricted T-cell responses prevent generation of paternal-antigen directed cell-mediated immunity.