Transcytosis of IgG

IgG molecules undergo transcytosis in syncytiotrophoblasts, specialized epithelial cells of the placental villi. IgG binds to FcRn receptors at the apical side, is internalized through pinocytosis, transported across the cell via endosomes, and released at the basolateral membrane into fetal blood. This process begins around 13 weeks of gestation and increases with gestational age.

FcRn receptors bind IgG molecules at the apical surface of syncytiotrophoblasts, initiating transcytosis. The acidic environment of early endosomes maintains the IgG-receptor complex binding. At the near-neutral pH of the basolateral membrane, IgG releases from FcRn receptors and enters fetal blood. This pH-dependent binding mechanism ensures efficient, directional transport across the placental barrier.

After pinocytosis, IgG-FcRn complexes enter a transport carrier that fuses with the early endosome, where acidic pH preserves receptor binding. A second transport carrier moves the complex to the recycling endosome. Finally, a third transport carrier delivers the complex to the basolateral membrane for release. This multi-step routing through recycling endosomes and transcytosis ensures precise cellular trafficking.

IgG undergoes transcytosis from apical to basolateral in syncytiotrophoblasts, while IgA moves from basolateral to apical in intestinal epithelial cells. IgG binds FcRn receptors; IgA binds poly-IgA receptors. Both use recycling endosomes but travel opposite directions. This directional difference reflects their distinct physiological roles in maternal-fetal immunity versus intestinal mucosal defense.

The acidic environment within early endosomes stabilizes IgG binding to FcRn receptors, maintaining the complex during transport. When the IgG-receptor complex reaches the basolateral membrane, the near-neutral pH causes IgG to dissociate from FcRn receptors. This pH-dependent release mechanism ensures IgG enters fetal blood only after crossing the epithelial barrier, preventing premature release.

Maternal IgG transferred through transcytosis offers protective immunity against infections during fetal development and early infancy. Vaccines administered during pregnancy, such as tetanus toxoid or inactivated influenza vaccines, stimulate IgG production that crosses the placenta via transcytosis. These maternal antibodies protect both mother and neonate from specific infections until the infant develops its own immune response.

Syncytiotrophoblasts are specialized epithelial cells forming the placental villi that mediate maternal-fetal exchange. These cells express FcRn receptors and perform transcytosis of IgG from maternal to fetal blood. Their unique polarized structure, with distinct apical and basolateral membranes, enables directional transport of IgG across the placental barrier while maintaining selective permeability.