For the last 40 years it has been considered that during pregnancy fetal antigens are expressed in a different manner in comparison with other grafts. Current data indicates that the acceptance of the fetus by the mother depends on the lack of expression of the polymorphic antigens and the production of hormones that act as immunological suppressors. During pregnancy there exists immunological recognition of the trophoblast antigens, but these antigens are not polymorphic and thus do not permit the identification of cytolytic T and natural killers cells. This structure also produces hormones that contribute to the diminished production and proliferation of T cells. It has been demonstrated that there exists an increase in the production by the trophoblast of complement inhibitors (DAF, CD46) and in the secretion of hormones such as progesterone, alpha-fetoprotein, steroids and prostanglandins. The identification of these immunological factors and mechanisms may be fundamental in the search for treatment regimens for such illnesses as cancer, infertility, spontaneous abortions, transplant rejection and graft versus host disease.