The conceptus is considered a semi-allograft because of the presence of paternal HLA-C molecules. These alloantigens can be processed by maternal antigen presenting cells, which present them to specific maternal CD4+ T cells. After activation, the maternal CD4+ T cells can become effector decidual CD4+ T cells, which are able to release various cytokines. Th1-type cytokines (IFNgamma) that promote allograft rejection may compromise pregnancy, whereas the Th2-type cytokines (IL-4, IL-10) that inhibit Th1 responses, promote allograft tolerance and therefore may improve fetal survival. A collaborative interaction between the decidual natural regulatory T cells, CD4+ CD25+ Foxp3 T cells and NKT cells in preventing fetal allograft rejection is suggested.