Neonatal Fc receptor (FCGRT) is kind of MHC class-I receptor. The neonatal Fc receptor (FCGRT) was first discovered in rodents. Further studies revealed a similar receptor in humans. In rodents, neonatal Fc receptor (FCGRT) in the neonatal gut epithelium and fetal yolk sac transports maternal IgG to the neonate and in humans neonatal Fc receptor (FCGRT) mediates maternal-fetal IgG transport across the placenta.
Neonatal Fc receptor (FCGRT) mediates transcytosis of IgG across absorptive epithelia in a variety of adult human tissues and cell lines including intestine, kidney, and lung. Neonatal Fc receptor (FCGRT) has also been localized to white blood cells, including monocytes and macrophages and has been proposed to play an important role in IgG-mediated phagocytosis in neutrophils.
Neonatal Fc receptor plays a role in monitoring IgG turnover. Neonatal Fc receptor binds IgG at acidic pH of (<6.5) but not at neutral or higher pH. Therefore, Neonatal Fc receptor can bind IgG from the intestinal lumen (the inside of the gut) at a slightly acidic pH and ensure efficient unidirectional transport to the basolateral side (inside the body) where the pH is neutral to basic (pH 7.0–7.5). Neonatal Fc receptor also plays a role in adult salvage of IgG through its occurrence in the pathway of endocytosis in endothelial cells. Fc receptors in the acidic endosomes bind to IgG internalized through pinocytosis, recycling it to the cell surface, releasing it at the basic pH of blood, thereby preventing it from undergoing lysosomal degradation. This mechanism may provide an explanation for the greater half-life of IgG in the blood compared to other isotypes.