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Abstract
The immunological events of pregnancy appear to be extremely complex. Numerous possibilities have been suggested as to why the semi-allogeneic fetus is not rejected by the mother. The suggestion that there is a lack of fetal antigen expression to activate maternal cells appears to be true. However, this lack of antigen stimulation of maternal cells is not due to an anatomical separation of fetal and maternal cells. The fetal trophoblast cells in contact with maternal (immune) cells do not express MHC Ia antigens and are therefore not recognized as non-self by maternal T lymphocytes. To escape lysis by uNK cells, the trophoblast cells express the MHC Ib antigens; HLA-E and HLA-G. Moreover, if immune cells in the presence of the trophoblast cells still become activated, the trophoblast cells are able to induce apoptosis in these activated immune cells, since they express apoptosis-inducing ligands, such as (FasL) and (TRAIL).