Pregnancy is a cooperative interaction between the mother and her fetus, allowing survival and normal growth of the fetus. Successful pregnancy remains a fascinating phenomenon as it resists the immunological rules of rejection. Immunological recognition of the fetus is vital for maintenance of gestation. The maternal immune system undergoes changes that lead to tolerance of the fetus. Inadequate recognition of fetal antigens may cause abortion. In fact, fetal cells express paternal alloantigens that are not recognized as foreign by the mother. A special balance between lymphocytes is present at the feto-maternal interface to control the immune response. In addition, placental trophoblasts act as a physical barrier and exert an immunoregulatory function. Trophoblast cells regulate the expression of human leucocyte antigens. Dysfunction of these cells leads to morphological and functional alterations of the feto-maternal barrier as well as to recurrent spontaneous abortions. Uterine natural killer cells are appropriate residents of the materno-fetal interface to support the adaptation of the blood vessels of the pregnant uterus and regulate trophoblast invasion into the decidua and myometrium. Cytokines are involved at the feto-placental unit by adapting normal T-cell trafficking and modulating the inflammatory process. This study discusses the complex immunological aspects of immune tolerance and the balance of immunity in pregnancy in terms of the role of the human leucocyte antigen, placental trophoblasts, maternal immunosuppression, immune cells, cytokines and immunoregulatory molecules at the feto-maternal interface.