Abstract: The human gestation is a semi-allograft and hence antigenically foreign to the mother. Therefore, the process of implantation may include mechanisms to prevent allograft rejection, but once the immunological tolerance becomes imbalanced, pathological pregnancy, such as spontaneous abortion could occur. Unexplained recurrent spontaneous abortion （ＵＲＳＡ） is thought to be caused by immunological rejection of the fetus by the mother. CD4+ T cells which include Th1, Th2, regulatory T cells （Ｔｒｅｇ） ａｎd Th17 cells, play important roles in allograft rejection. Maternal peripheral blood and decidual CD4+ T cells augmentation have been implicated in URSA. Mechanisms of immune tolerance is in the deepening，from the Th1/Th2 immune balance to the negative regulation of Treg cells. Recently, the Th17/Treg balance, has been described to paly an important role in the immunological rejection. In this paper the role of CD4+ T cells in URSA is reviewed.

Key words: Abortion, recurrent, Th1 cells, Th2 cells, T-lymphocytes, regulatory