Recent advances in the understanding of immunological events during pregnancy offer new diagnostic and therapeutic approaches to couples suffering from unexplained recurrent spontaneous abortion. In terms of immunology the success of pregnancy means, that the fetus as a semi-allogeneic graft is not rejected by the mothers/recipients immune system. Beside a variety of specific and non-specific mainly locally acting substances a process of active tolerance induction is responsible for fetal survival. In response to paternally inherited fetal antigens the maternal immune system produces so-called blocking factors. In most women with unexplained recurrent spontaneous abortion these blocking factors cannot be detected. By immunization with paternal or third party lymphocytes the maternal immune system can be stimulated to produce blocking factors and therefore to protect a conceptus from rejection.