Does IgG Cross the Placenta?
Yes, IgG is the only antibody class that crosses the placenta, providing passive immunity to the fetus and newborn. 1
Mechanism of Placental Transfer
Maternal IgG antibodies cross the placenta through an active transport mechanism mediated by the neonatal Fc receptor (FcRn) expressed on syncytiotrophoblast cells. 1 The process occurs as follows:
- IgG antibodies are taken up into endosomes within syncytiotrophoblast cells and bind to FcRn 1
- Following acidification of the endosome, IgG antibodies are transcytosed to the fetal side of the syncytiotrophoblast 1
- The endosome fuses with the syncytiotrophoblast membrane, releasing IgG into fetal circulation 1
- The higher physiological pH in fetal circulation promotes dissociation of IgG from FcRn 1
Timing and Efficiency of Transfer
IgG transfer increases linearly throughout pregnancy, with the largest amount transferred during the third trimester. 2
- Maternal IgG concentrations in fetal blood increase from early in the second trimester through term 3
- Most antibodies are acquired during the third trimester 3
- Preterm infants receive less maternal IgG and are therefore more susceptible to infections 4
IgG Subclass Transfer
Not all IgG subclasses cross the placenta with equal efficiency:
- IgG1 is the most efficiently transported subclass 4, 2, 3
- IgG4 is transported moderately well 2
- IgG3 has intermediate transport 2
- IgG2 is the least efficiently transported subclass 4, 2, 3
This differential transfer explains why preterm newborns are especially susceptible to encapsulated bacteria (which require IgG2 for protection). 4
Clinical Implications
The ability of IgG to cross the placenta forms the basis for maternal immunization strategies to protect newborns:
- Maternal vaccination during pregnancy transfers protective antibodies to the infant, narrowing the "window of vulnerability" before infant vaccination series are complete 1
- Examples include tetanus, diphtheria, pertussis, and influenza vaccines 1
- Vaccines are typically not administered to infants until at least 6 weeks of age and require multiple doses for full protection 1
Important Caveats
Harmful antibodies can also cross the placenta:
- In mothers with autoimmune or alloimmune disorders, pathological antibodies may cause fetal or neonatal harm 5
- RhD alloimmunization results from maternal anti-D IgG antibodies crossing the placenta and causing hemolytic disease in RhD-positive fetuses 6, 7
- Autoantibodies in maternal autoimmune diseases can cause transitory autoimmune disease in the neonate 8
Therapeutic monoclonal antibodies (IgG1 molecules) used to treat conditions like inflammatory bowel disease also cross the placenta, with greatest transfer in the third trimester. 2