Transplacental transport of maternal immunoglobulin G (IgG) to the developi
ng fetus is extremely important in the protection of the newborn from infec
tion. Although the exact mechanisms of the selective and active transfer of
IgG across the placental barrier are not fully understood, receptors for t
he Fc part of IgG (Fc gamma Rs) in the placenta are believed to play a key
role. Several known Fe receptors, Fc gamma RI, Fc gamma RII, Fc gamma RIII
and FcRn (neonatal FcR), demonstrate heterogeneous expression patterns in p
lacenta. Immunohistochemical analysis shows the expression of Fc gamma RI o
n Hofbauer cells in stromal tissue, Fc gamma RII on Hofbauer cells and feta
l blood endothelium, Fc gamma RIII on Hofbauer cells and trophoblasts, and
FcRn on syncytiotrophoblasts and endothelial cells. Recent studies provide
evidence for important associations among these receptors and transcytosis
of IgG, as well as scavenger mechanisms for clearing immune complexes in th
e placenta during pregnancy.