Major histocompatibility complex (MHC)-mediated immune regulation of decidual leukocytes at the fetal–maternal interface

Self and non-self recognition is the key mechanism by which the immune system determines whether or not to mount an immune response. During pregnancy the maternal immune system must tolerate the persistence of non-self semi-allogeneic fetal cells in the maternal tissue. Although many mechanisms have been shown to contribute to the prevention of a destructive maternal immune response to fetal cells, the immune acceptance of the allogeneic fetus in pregnancy largely remains an immunological paradox (Billingham et al., 1953). The aim of this review is to describe the expression of the polymorphic histocompatibility antigens at the fetal–maternal interface, their interaction with maternal leukocytes and their possible roles in immune regulation at the fetal–maternal interface during human pregnancy.