[Two individuals under the same skin--immune mechanisms in pregnancy]

The fetus may be regarded as an allograft in the maternal organism. This paper gives an overview of pregnancy-associated immune mechanisms, based on the literature and studies performed by the authors. During implantation, maternal tissues are invaded by fetal trophoblasts expressing HLA-G, a trophoblast-specific variant of HLA Class I antigens. Recognition of HLA-G stimulates uterine natural killer cells to cytokine production, by which an intrauterine immunosuppression is established. Development, growth and differentiation of placenta is regulated by the cytokines produced. Uterine leukocyte population and expression of cytokine receptors in placental tissues varies throughout gestation, and the complex interplay between trophoblasts and uterine cells, involving a number of cytokines, cytokine receptors, adhesion molecules, enzymes and hormones, changes with gestation. Some cytokines, such as tumour necrosis factor and interleukin-1, may threathen the reproductive process and fetal well-being in high doses. A tight regulation of cytokine activities is probably obtained by the observed upregulation of endogenous cytokine buffer mechanisms in pregnancy. The reproductive success and phenomenons like implantation, placental growth and development, maintenance of pregnancy and delivery, appear to rely on complex, gestational age related interplay between cells of fetal origin and the maternal immune system.