In line with the above, it has been shown that among OD pregnancies, the degree of HLA mismatch between mother and fetus is correlated with a higher number of maternal decidual-activated CD4+ Treg cells (101–104), a reduced number of tissue macrophages (105, 106), and the development of gestational hypertension and preeclampsia (107, 108). This evidence concerns the gene CD4 and preeclampsia.