The neonatal Fc receptor, also known as the Brambell receptor, is a protein that in humans is encoded by the FCGRTgene.
The neonatal Fc receptor is an Fc receptor which is similar in structure to MHC class I. It was first discovered in rodents as a unique receptor capable of transporting IgG from mother's milk across the epithelium of newborn rodent's gut into the newborn's bloodstream. Further studies revealed a similar receptor in humans. In humans, however, it is found in the placenta to help facilitate transport of mother's IgG to the growing fetus and it has also been shown to play a role in monitoring IgG turnover. FcRn binds IgG at acidic pH of (<6.5) but not at neutral or higher pH. Therefore, FcRn 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).
This 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 (3 weeks). It has been shown that conjugation of some drugs to the Fc domain of IgG significantly increases their half-life, likely through this mechanism. There are several drugs on the market that have the Fc portion fused to the effector protein in order to increase its halflife. They are: Amevive (alefacept), Arcalyst (rilonacept), Enbrel (etanercept), Nplate (romiplostim), Orencia (abatacept) and Nulojix (belatacept). Specially in mice, it is also found in intestinal epithelium, where pH is low and IgG from the mother could bind and pass through endosomes to the baby blood in its first weeks of life.
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