Reproductive immunology refers to a field of medicine that studies interactions (or the absence of them) between the immune system and components related to the reproductive system, such as maternal immune tolerance towards the fetus, or immunological interactions across the blood-testis barrier. The concept has been used by fertility clinics to explain fertility problems, recurrent miscarriages and pregnancy complications observed when this state of immunological tolerance is not successfully achieved. Immunological therapy is a method for treating many cases of previously "unexplained infertility" or recurrent miscarriage. The immune system and pregnancy The immunological system of the mother plays an important role in pregnancy considering the embryo's tissue is half foreign and unlike mismatched organ transplant, is not normally rejected. During pregnancy, immunological events that take place within the body of the mother are crucial in determining the healthiness of both the mother and the fetus. In order to provide protection and immunity for both the mother and her fetus without developing rejection reactions, the mother must develop immunotolerance to her fetus since both organisms live in an intimate symbiotic situation. Progesterone-induced-blocking factor 1 (PIBF1) is one of the several known contributing immunomodulatory factors to play a role in immunotolerance during pregnancy. The placenta also plays an important part in protecting the embryo for the immune attack from the mother's system. Secretory molecules produced by placental trophoblast cells and maternal uterine immune cells, within the decidua, work together to develop a functioning placenta. Studies have proposed that proteins in semen may help a person's immune system prepare for conception and pregnancy. For example, there is substantial evidence for exposure to partner's semen as prevention for pre-eclampsia, a pregnancy disorder, largely due to the absorption of several immune modulating factors present in seminal fluid, such as transforming growth factor beta (TGFβ). Insufficient immune tolerance An insufficiency in the maternal immune system where the fetus is treated as a foreign substance in the body can lead to many pregnancy-related complications. Rh disease, or Rh isoimmunization, occurs when the maternal immune system develops antibodies that recognizes fetal red blood cells as foreign. This can lead to a number of potentially dangerous consequences to the fetus including hemolytic disease due to the destruction of red blood cells, kernicterus, or even death. Treatment with anti-D immunoglobulin has been studied extensively on the prevention of Rh disease. However, there has been no conclusive evidence that treatment with