A review analyses the findings of studies according to which immunoregulation in pregnancy after in vitro fertilization (IVF) with the use of donor oocytes differs from such in natural pregnancy. The number of HLA mismatches influences the number of activated T cells in peripheral blood. In pregnancy after IVF and oocyte donation the mother is exposed to alien cells and antigens, i.e. the situation is comparable with blood transfusion and organ transplantation. Th1 and Th2 cells become hyperactivated. This fact is suggestive that allogeneic foetus induces an additional mechanism contributing to preservation of pregnancy. However, the mechanisms that are involved in this altered immune response have not been fully studied yet.