New functions of the MHC class I-related Fc receptor, FcRn

FcRn was originally identified as the receptor responsible for IgG binding to the intestinal epithelium of neonatal rats and mice. FcRn transports IgG from milk across the intestinal epithelial cells of the suckling animal. Subsequently, FcRn was detected in tissues involved in the transmission of IgG from mother to fetus: rat fetal yolk sac, mouse fetal yolk sac and human placental syncytiotrophoblast. In addition, FcRn mRNA has been detected in many tissues of adult rats, mice and humans, and FcRn is present in several adult tissues and in cell lines. The selective protection from catabolism of IgG compared with other immunoglobulins is lost in mice that lack functional FcRn. One function of FcRn in tissues that do not transport IgG, and beyond the perinatal period, is thus to rescue circulating IgG from degradation. These recent observations reveal a more widespread use of FcRn than had been supposed.