Maternal cell-mediated sensitisation to paternal HLA may occur, but is not a regular event in normal human pregnancy.

Maternal cell-mediated sensitisation to paternally-inherited fetal HLA was studied in 10 primigravid and 9 multigravid women. One woman in each group was found to have circulating cytotoxic effector cells specific for paternal lymphocytes, but both had a normal pregnancy outcome. These results show that sensitisation is not a regular event in pregnancy and that there must therefore be a block in the afferent (recognition) phase of the mother's immune response. Where sensitisation does occur there must also be a block on the efferent (effector) arm of the response to prevent rejection of the fetus. The restricted distribution of major histocompatibility complex antigens on trophoblast and the function of the placenta as a barrier between maternal and fetal immune compartments can explain the occurrence of both types of blockade.