Immune responses play an important role in various reproductive processes, including ovulation, menstruation and parturition. Clearly, during pregnancy, when the mother must accept a semi-allogeneic fetus, immune responses also play a very important role. This was first recognized by Medawar in 1953, when the concept of the fetal allograft was presented in order to explain the immunological relationship between mother and fetus. Since then, the immunology of pregnancy has been the leading subject within reproductive immunology research. Yet, the question of why the semi-allogeneic fetus is not rejected by the mother remains unresolved. The present review provides an update of current knowledge on the subject of the so-called 'immunological paradox of pregnancy'.