Immunology and Reproduction
The relationship between a living organism and its environment is based on a tightly regulated balance between symbiosis and competition. The complexity of an organism is directly correlated to the sheer volume of challenges presented to its continued fertility. The systems in higher organisms that function to protect against biological insults are collectively referred to as the immune system. The immune system may act as friend or foe. Immune systems of higher vertebrates can be roughly subdivided into two major components: a phylogenetically older innate immune system, and the more recently evolved adaptive immune system. The diversity of environments that humans occupy is staggering; thus the human body has remarkably elaborate mechanisms to protect the individual from foreign pathogens.During development, the innate and adaptive arms of the immune system are programmed and learn to distinguish self from nonself. Despite the safeguards of the immune system, some self-reacting lymphocytes are not inactivated or removed and can cause one of several illnesses we know as autoimmune diseases. Immunologists are well aware that if they understood all the mechanisms for tolerance induction, they might be able to prevent autoimmune diseases, and conversely, focus the potent tool-kit of immunity against foreign pathogens and the "altered self" of cancer.The failure of tolerance-induction mechanisms to prevent the recognition of "self" antigens as foreign by the immune system may result in an autoimmune response. Thus autoimmunity refers to an immune reaction of the body against substances that are normally present in the body. To understand autoimmunity, it is important to first understand tolerance. Tolerance can best be defined as a state of antigen-induced immunologic unresponsiveness. (Autoimmunity can be considered a failure of tolerance.) Tolerance is not synonymous with generalized immunosuppression; it is antigen-specific and causes no impairment of the immune response to antigens other than the ones used to induce tolerance.Implantation is one of the most important aspects of pregnancy that immunologic studies have targeted. Implantation represents a critical developmental process in that it requires the interaction of immunologically and genetically distinct tissues. The immune system may influence pregnancy success or failure during any of the critical steps of implantation. Embryo attachment occurs only during the "implantation window," the time period when the endometrial epithelium is receptive and the embryo is hatching and competent for attachment. Failure of this synchronization precludes success, as demonstrated in human studies of implantation.Studies of the karyotype on products of conception from early recurrent pregnancy loss have shown that many of these losses are chromosomally normal human embryos. Human preimplantation embryos express major histocompatibility antigens theoretically capable of inducing an immune response, but the role of these antigens in pregnancy has not been delineated. It is possible that maternal immune responses play a role in the failure of implantation.
