In humans, maternal immunoglobulin G or IgG is transported from the placenta to the fetal blood through transcytosis. IgG molecules undergo transcytosis in syncytiotrophoblasts, specialized epithelial cells of the placental villi. IgG molecules bind to FcRn receptors at the apical side of syncytiotrophoblasts. The IgG-FcRn receptor complex is internalized through pinocytosis and sorted into a transport carrier. The transport carrier first fuses with an early endosome where the acidic environment keeps the IgG bound to the FcRn receptor. The IgG-receptor complex is then transported through a second transport carrier to the recycling endosome. Finally, a third transport carrier delivers the IgG-receptor complex to the basolateral membrane on the fetal side of the epithelial cell. The near-neutral pH of the basolateral membrane causes the release of the IgG from the FcRn receptor into the neonate’s bloodstream.