The neonatal Fc receptor (abbreviated as FcRN) is a heterodimeric receptor that binds the Fc-regions of antibodies at low pH in endosomes but not at neutral pH (except IgG3). This is critical for IgG recycling responsible for long half-life. Schoch et al. (1) showed that the Fv-region (more specifically the light chain) also binds FcRn, and that slow dissociation can shorten antibody half-life.
Structure
FcRn is a heterodimeric protein with an glycosylated class I MHC-like protein alpha subunit, and an immunoglobulin beta subunit.
1.
Schoch A, Kettenberger H, Mundigl O, Winter G, Engert J, Heinrich J, et al. Charge-mediated influence of the antibody variable domain on FcRn-dependent pharmacokinetics. Proceedings of the National Academy of Sciences. 2015;112(19):5997–6002. Available from: https://doi.org/10.1073/pnas.1408766112