Recent data have indicated that the major histocompatibility complex (
MHC) class I-related receptor FcRn plays a role in regulating serum Ig
G levels, in addition to transferring maternal IgGs across the rodent
neonatal gut. The isolation of a human homolog of FcRn from placenta s
uggests that the studies in rodents have relevance to understanding si
milar processes in humans. This has implications for the engineering o
f improved antibodies for therapy.