Actually, the rejection would be mediated by foreign MHC! i.e., paternal
MHC is incompatible (usually) with maternal, so why is the baby not
rejected by Maternal T cells?
The answer is that the outer layer of the embryo consists of a cell type
called trophoblasts. Trophoblasts are unique in that they do not express
MHC! The downregulation of MHC expression is achieved by specific
methylation of the genes encoding MHC. The lack of MHC expression keeps
the embryo from being rejected, and leaves it susceptible only to
Ig-mediated responses (i.e., mismatched RH).
mr
