Introduction

Implantation of the developing embryo into the wall of the maternal uterus with subsequent development of the placenta is a critical stage of human pregnancy. The close apposition of placental and uterine tissues creates an immunological dilemma. The placenta, being a hybrid between paternal and maternal genomes, has the potential to express paternal antigens and thus could potentially be recognised as nonself by the maternal immune system. In transplantation, such recognition of nonself results in rejection of the grafted tissue. If similar mechanisms operate in the uterus then reproductive success must rely on some form of immunological accommodation of the mother to placental tissue. The subject of this review is to consider the unique immunological environment of the human uterus, which permits this accommodation.

Immunological models

The classical self/nonself model formulated by Burnet and Medawar proposed that each lymphocyte expresses a single receptor for a foreign antigen and signalling through this triggers an immune response (Figure 2.1). Lymphocytes with receptors for nonself in the form of pathogens and allogeneic cells will be present and, if these are encountered, elimination of the infectious agent or rejection of the allograft results. The random generation of these clonally distributed receptors by gene rearrangement during fetal development means that the T and B lymphocytes, which acquire receptors reactive to antigens present in the fetal thymus, need to be deleted. In this way, tolerance to self and infectious or other stimuli encountered before birth is acquired.