Infertility may be the result of immunologic factors that interfere with the development of the early placenta and the fetus. These immunologic problems are frequently unrecognized. They can be the cause of unexplained or longstanding infertility in a couple who have failed both standard fertility treatments and even high-tech assisted reproductive technologies. Women with immunologic problems may develop so-called phospholipid (foss-foh-LIH-pid) antibodies, which can attack the early growth of a placenta when roots are being formed in the lining of the womb. In some cases, the woman makes an insufficient quantity of 'blocking' antibodies designed to block an allergic response from the proteins and substances in the baby contributed by the father. Fortunately, immune treatments are now available for virtually all these immunological problems causing infertility.