The long-standing question of pregnancy immunological paradox has been generating renewed interest. Recent insights have emerged from studies in pregnant mice and humans demonstrating a number of mechanisms that prevent potentially harmful effects of maternal anti-paternal allo-antibodies (complement inhibition, partial deletion of maternal B cells specific of paternal antigens), cytotoxic CD8+ T cells (lack of HLA-A and HLA-B expression on trophoblast, local immunosuppressive molecules, transient tolerance of paternal allo-antigens specific T cells) and uterine NK cells directed against fetal-derived trophoblast cells (limited NK cytotoxic potential, trophoblast resistance to NK killing). Interestingly, it appears that not only decidual NK cell/trophoblast interactions are not harmful for the fetus but are beneficial for the placental vascularization and its subsequent development. A recent report has indeed demonstrated that during pregnancy most of the combinations of uterine KIR (killer cell immunoglobulin-like receptor) NK cell receptors and fetal HLA-C molecules expressed by trophoblast led to normal pregnancies, whereas mothers lacking activating KIR of the AA genotype when the fetus possessed HLA-C of the C2 group were at a greatly increased risk of severe preeclampsia pathology.