The conceptus may be considered as a sort of semi-allogenic graft for the maternal organism, since it shares a half of genomic complement with the father. Nevertheless, its rejection does not take place physiologically during a pregnancy. The mechanisms resulting in the maternal immune tolerance versus the conceptus are not yet completely clarified. Such mechanisms are probably multiple and interacting with each other. In animal and in vitro studies provide evidence suggesting that the following factors are important in producing the maternal immune tolerance: the anatomical position of the fetus; the absence of expression of the class I and II Major Histocompatibility Complex (MHC) molecules in trophoblast tissues; the activity of blocking antibodies; a modification of the immune response; the fetal-placental production of immunosuppressive hormones and substances. Amongst pregnancy-related changes in the immune response, a reduced Natural Killer (NK) cell activity and an increased synthesis of Th2 cytokines (which inhibit the cell-mediated immunity) with an altered Th1/Th2 balance appear to be remarkably important. With regard to fetal-placental hormones, progesterone seems to exert an important immunosuppressive influence mediated by the protein named "Progesterone Induced Blocking Factor" (PIBF). Nevertheless, the real contribution of each of the above mentioned mechanisms still remains to be elucidated in humans.