Autoimmunity and Pregnancy Loss

David A. Clark

In an outbred population such as humans, tissue transplanted from one individual to another is usually rejected by the recipient's immune system. This occurs because each individual expresses on its cells individual-specific antigens (alloantigens) that elicit transplant rejection (transplantation antigens) by an unrelated recipient. The most important antigens are coded by genes in the major histocompatibility complex (MHC) and are divided into class I and class II sets. Class I alloantigens are called HLA-A, -B, and -C, and are expressed on all somatic nucleated cells; class II antigens are expressed primarily on lymphomyeloid cells (lymphocytes and macrophages) of the immune system. MHC antigens did not evolve for the purpose of frustrating transplantation surgeons, but rather as a mechanism by which cells could distinguish "self" from "not self". From the time of development of primitive multicellular organisms, somatic cells could recognize each other, and when a foreign (allogeneic) cell was present, it could not be excluded...