Pregnancy is a unique immunological situation in which 2 allogeneic organisms live in intimate symbiosis without developing rejection reactions. At different locations, interfaces exist between mother and foetus with direct contact between both individuals: 1) maternal blood surrounds foetal villi, which are covered with syncitiotrophoblast cells; 2) cytotrophoblast cells invade the decidua, in which they touch tissue lymphocytes; 3) trophoblast cells, which substitute endothelium of maternal arterioles filled with maternal blood; and 4) trophoblast particles, which are expressed from syncytiotrophoblast and circulate within the maternal blood until they settle in the lung capillaries, where they become degraded by alveolar macrophages. Several factors are known which support the specific immunotolerance of the mother to her foetus and are focussed by current research in reproductive immunology. One of these factors is progesterone-induced blocking factor (PIBF). Originally, it was discovered as a 34 kDa protein, which is released from lymphocytes of healthy pregnant women under the influence of progesterone. PIBF has immunomodulatory functions in vivo and in vitro, which are important for the establishment of immunotolerance between mother and foetus and, thereby, for the regular course of pregnancy. Finally, during the last years, several tumours have been identified to produce PIBF, which supports their immune escape and which may have the potential to become a novel tumour biomarker and which may lead to the development of new therapeutic strategies.