In recent years it has come to be generally recognized that, directly or indirectly, immunology intrudes into nearly every aspect of human reproduction. The discovered importance of the natural transfer of immunity from mother to offspring, the ontogeny of the immune response, and the recognition of pregnancy as an example of the mother-embryo interaction, are only a few components of the multifaceted field known as the immunology of reproduction. In this book, we have compiled material, obtained in our laboratories and gleaned from the modern publications illustrating ongoing research in the field of reproductive immunology on the immune system in human intrauterine development. Many new approaches have been postulated and published in recent years, enough to warrant bringing them together in one place. In this book, we focus our attention mainly on the application of humoral and cellular changes that describe the role of immune systems in human intrauterine development. A substantial area of the reproductive immunology deals with the issue of the maternal-fetal conflict and the survival of the embryo/fetus as an allograft. Although much efforts has been invested in describing different aspects of this problem, relationships between the pregnant mother and her developing fetus are not yet fully understood from an immunological point of a view. The role of the fetal immune system in its survival and in the maintenance of pregnancy have only been scanty studied, especially with respect to the secretory immune system. General skepticism has been developed regarding the diagnosis and treatment of recurrent spontaneous abortions. In our research, we have described changes in the secretory immune system of the embryo/fetus and mother as possible reasons for such abortions and we hope that these findings will yield a better understanding of the mechanism underlying this phenomenon. For many years, our laboratories have been studying the pathology of fetuses and the immune response of their lymphoid systems to experimentally induced disorders in laboratory animals (mice and rats) and under some diseases in humans, such as hemolytic disease of fetuses and newborns, sepsis of fetuses, and others. In the last decade, we have concentrated our efforts on studying the immune protection of human embryos and fetuses under non-antigen and antigen-induced diseases, studying the secretory (mucosal) immune system. The changes in the different types of immune systems in the intrauterine-developing organism in normal pregnancy, under non-infectious pathology and under infectious effects as well as under mother-fetus immune conflict are discussed in this book. The protective role of mononuclear phagocytes and the non-villous infiltrating trophoblast is analyzed. The pathological morphology of some immune and inflammatory processes, including massive apoptosis of embryonic cells, is also discussed. We hope that the problems described in this book will be of interest to people with various professional orientations, such as physicians and researchers in the field of obstetrics and gynecology, reproductive immunology and intrauterine human development, as well as medical students. For the clinician and scientists in other areas of medicine, pregnancy provides an area in which many immunological ideas can be tested. This is particularly true now that the impact of pregnancy impacts on the maternal immune system is becoming better understood.