The neonatal Fc receptor (FcRn) was first found to be responsible for transport of antibodies of the IgG class from the mother to the fetus or neonate, as well as for protecting IgG from intracellular catabolism. However, it has now become apparent that the same receptor also binds albumin and plays a fundamental role in homeostatic regulation of both IgG and albumin. FcRn is expressed in many different cell types and organs at diverse body sites.
Albumin is the most abundant protein in the blood and a versatile transporter of small molecules such as fatty acids, hormones and drugs. An in-depth characterization of how FcRn binds and regulates the transport of albumin is necessary to get a complete understanding of its biodistribution and functions. Importantly, such knowledge may also guide development of new drugs, as IgG and albumin are increasingly utilized in therapy. Here I will discuss our current structural and biological understanding of the interactions between FcRn and albumin, knowledge that may inspire development of novel engineered albumin fusions and conjugates with therapeutic value.