Maternal tolerance of the fetal allograft could be the result of the integration of numerous mechanisms promoted by different cells present in the decidua. Decidual macrophages and dendritic cells, which are found in close association with T lymphocytes are the most potent activators of T lymphocyte responses and could play a sentinel function for the immune system, initiating antigen-specific T cell responses to fetal antigens. T cell cytokines produced in response to fetal molecules could have a role in the maintenance or in the failure of pregnancy. The levels of LIF, IL-4, IL-10 and M-CSF produced by decidual T cells of women suffering from unexplained spontaneous abortion are lower than those of normal pregnant women indicating that these cytokines may contribute to the maintenance of pregnancy. T cells from the cumulus oophorus surrounding the preimplantation embryo produce LIF and IL-4. These findings suggest that cytokines produced by maternal T cells create a suitable microenvironment for preimplantation embryo development and maintenance of pregnancy. T cell cytokine profile could be modulated by the hormones present in the microenvironment of T cells: high doses of progesterone present at fetomaternal interface and in the cumulus induce the production of IL-4 and LIF, whereas relaxin induces IFN-gamma production.