Why aren't embryos immunologically rejected by their mothers?

Abstract

Just over 50 years ago, Peter Gorer discovered the major histocompatibility complex (MHC) of the mouse, the H-2 complex (Gorer, 1936a,b; 1937). Since that time, an MHC has been defined for most mammals, and for a variety of lower vertebrate organisms (reviewed in Klein, 1986). The MHC is a set of genes that encodes the information for proteins found on the surface of cells of each individual. The proteins are highly polymorphic, and each individual in the species has a particular set of proteins defining that individual’s tissue type or MHC phenotype. The products of the MHC genes are crucial for the control of a variety of biological functions, including cell-cell interactions in the immune response, tumor and graft rejection, and reproductive success. The best-studied MHC is that of the mouse. A diagram of the current view of the mouse MHC in shown in Figure 1. Extensive work on cloning and sequencing the genes of the H-2 complex, initiated in 1980, has shown that the H-2 complex encodes at least 50 genes (Steinmetz, 1987). The MHC genes code for three different types of proteins, designated Class I, Class II, and Class III. Class I molecules are membrane-bound glycoproteins, all of which have a similar structure, a 35-45,000 molecular weight heavy chain and a 12,000 molecular weight light chain. The heavy chain is encoded in the H-2 complex, but the light chain, 132 -microglobulin, is encoded on mouse chromosome 2. In most species, the Class I genes are clustered at the telomeric end of the MHC, but in the mouse, two Class I genes have been translocated to the centromeric end of the MHC gene complex.