FIGURE 2.

Timely shifting of Th1/Th2 and Treg/Th17 balance is of utmost importance for a successful pregnancy. During the implantation phase, Th1 cells are infiltrated at the decidua and regulate trophoblast invasion by participating in tissue remodeling and angiogenesis. Th17 and Th22 cells are present in the decidua but not enriched. Th17 cells involve in neutrophil infiltration and induction of protective immunity against extracellular microbes in the uterus. Th22 cells also protect trophoblast cells from pathogens and inflammatory immune responses from intrauterine infection. Decreased Th22 cells induce dysfunction of decidual stromal cells and NK cells. Hence, Th22 cells are keenly associated with Th1/Th2 balance. Treg and Th2 cells repress excessive Th1 and Th17 immunity. When the implantation phase is over, Th2 and Th9 cells become dominant and promote allograft tolerance. Tfh cells are usually enriched at late gestation, favoring humoral immunity and, thus, balancing Th1/Th2 immune responses.