IgG is Small Enough to Pass Through the Placenta
IgG (Immunoglobulin G) is the only antibody class that significantly crosses the human placenta, while IgA (Immunoglobulin A) does not efficiently transfer and remains primarily in maternal circulation. 1
Mechanism of IgG Placental Transfer
IgG antibodies cross the placenta through active transport mediated by the neonatal Fc receptor (FcRn) expressed on syncytiotrophoblast cells. 1, 2
The transfer process involves IgG binding to FcRn in acidified endosomes within the syncytiotrophoblast, followed by transcytosis to the fetal side where antibodies are released into fetal circulation at physiological pH. 1, 3
This active transport mechanism begins around week 13 of gestation and progressively increases until delivery, with the majority of transfer occurring during the third trimester. 1, 4
IgG Subclass Transfer Efficiency
IgG1 is the most efficiently transported subclass across the placenta, followed by IgG4, IgG3, and then IgG2 (which transfers least efficiently). 1, 4
Monoclonal IgG1 antibodies (such as infliximab and adalimumab) can achieve fetal cord blood levels up to fourfold higher than maternal blood levels when administered during pregnancy. 1
Why IgA Does Not Cross the Placenta
IgA does not undergo significant placental transfer because it lacks the structural requirements for FcRn-mediated transport. 2
While trace amounts of IgA may be detected in coelomic fluid in early pregnancy (at concentrations 128 times lower than maternal serum), this represents minimal transfer and does not provide meaningful fetal protection. 5
IgA's primary role is in mucosal immunity and it remains in maternal circulation and secretions rather than crossing to the fetus. 2
Clinical Implications
Maternal vaccination during pregnancy leverages IgG transfer to provide passive immunity to newborns during their vulnerable first months of life before their own immune system matures. 1
This mechanism is particularly important because infants cannot receive many vaccines until at least 6 weeks of age and require multiple doses for full protection. 1
The selective transfer of IgG (but not IgA or IgM) explains why maternal immunization strategies focus on generating IgG responses to protect the fetus and newborn. 1, 2, 4