Mechanisms explaining maternal tolerance of her semiallogeneic histoincompatible fetus have been proposed to include a number of unique signaling molecules including CD200, novel MHC class I-b molecules such as HLA-G and HLA-E, Th2,3 cytokines, apoptosis-inducing molecules such as FASL, and indoleamine 2,3-dioxygenase. Novel CD4+ CD25+ Treg cells and gammadelta T cell receptor-positive regulatory cells appear to play key roles in responding to and in generating signals. This chapter will critically review current data concerning the mechanisms and relevance of the various proposed mechanisms.
