Transcytosis of immunoglobulin G

Immunoglobulin G (IgG) is transported across several epithelia, resulting i n the transfer of passive immunity from mother to offspring. Since the newb orn lacks active immunity, it is dependent on the transfer of maternal IgG, which can either occur prenatally or after birth. In both cases FcRn, an I gG Fc receptor homologous to major-histocompatibility class I molecules, is thought to mediate the transepithelial transport of IgG. Recently, a more general role of FcRn in IgG homeostasis has been postulated. However. it is only poorly understood how the receptor mediates these distinct IgG transp ort routes. In the present article we briefly review the structure and func tion of FcRn and discuss the evidence for its role in fetal and neonatal im munity.