Abstract: Immune factors and immunocytes play critical roles in the maintenance of pregnancy，abnormal immune response of the mother toward the fetus causes abortion. One kind of protein called progesterone-induced blocking factor （PIBF） is derived by lymphocytes of pregnant women. PIBF increases Th2 type cytokine production，induces Th1/Th2 balance bias to Th2 type cytokine，inhibits NK cells activity，reduces arachidonic acid release and promotes the synthesis of asymmetric antibodies to mediate the immunomodulatory and anti-abortive effects of progesterone. Women with threatened abortion or pre-term labour display reduced concentrations of PIBF than women with viable pregnancy. PIBF in peripheral blood lymphocytes，blood serum，and urine can be tested with ELISA，immunohistochemistry，western blot and flow cytometry. Progesterone and dydrogesterone might improve pregnancy successfully by inducing secretion of PIBF. In this review，we summarise the regulation of PIBF in pregnancy immunity.

Key words: Pregnancy, Abortion, Immunocompetence, Progesterone, Progesterone-induced blocking factor