The'foetal allograft'concept of the maternal-foetal relationship is currently being questioned. This is partly due to the increasing awareness of uterine large granular lymphocytes, a subset of natural killer cells found in the human endometrium at the time of placentation. Analogous cells are present in several species. The possible role of these cells in the regulation of the trophoblastmaternal interface is discussed here by Ashley King and Yung Wai Loke.

Granulated cells were noted in the pregnant uterine lining (decidua) of many species including rodents (granulated metrial gland (GMG) cells) and humans (endometrial granulocytes) many years ago 1, 2. These were subsequently identified as lymphoid cells, but detailed characterization by immunohistology and flow cytometry have recently shown that human uterine granulated cells have a distinctive surface phenotype reminiscent of CD3-natural killer cells/large granular lymphocytes (NK cells/LGLs)(Refs 3-5, Fig. i and Table 1). However, there are differences between uterine LGLs and peripheral blood NK cells. First, whereas most cir-