In the case of IgG, the receptor responsible for transcytosis of IgG into colostrum is referred to as FcRn, or the neonatal Fc receptor, because it was initially identified in the neonatal rodent intestine as the receptor responsible for the specific uptake of maternal IgG [123, 124] . The FcRn also has been implicated in the trans-placental transport of IgG in humans and other species [125] [126] [127] , which may involve an endocytic and transcytotic process [128] . Since its initial discovery, FcRn has been described in many tissues [122] . The receptor is a heterodimer composed of a membrane-bound α-chain similar to MHC class-1 molecules and a smaller MHC class-1 protein, β2-microglobulin [129] . Binding of IgG to FcRn is pH-dependent, with high affinity binding occurring at acidic pH, but only weak binding at neutral or basic pH [122] . This observation suggests that IgG taken up by the epithelial cells may bind to FcRn within an acidic environment in the endosomes. The precise mechanism of transport across the epithelial cell and release into the colostrum or milk remains to be demonstrated.