One of the great mysteries in modern immunology is how extraembryonic membranes escape rejection by maternal immune response, although they express paternal genes/anti-genes which should stimulate allogenic recognition and rejection. Generally, two theories try to explain the pregnancy phenomenon. One of the emphasizes the role of immunosuppressive reactions in the protection of the fetus. On the contrary, the "immunotropism" theory insists on the importance of mother's immune response to paternal antigens of the conceptus. Moreover, the last years abound in discoveries on molecules regulating cell-cell interactions at the level of the initiation and effector stage of the immune response. The best examples of such molecules could be extracellular matrix proteins, integrins, interleukins and various growth factors. The discussion on those molecules as regards their role in the protection of the fetus was the main aim of this article.