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Maternal antibodies
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A baby is born with defenses they did not make: their mother passed them along during pregnancy. Of all the classes of antibody, only one crosses the placenta in quantity, and that is IgG, immunoglobulin G.
It does not get across by filtration; it rides across on a transporter. It is called the neonatal Fc receptor, or FcRn, and it lives in the syncytiotrophoblast, the layer of placental cells that faces the mother's blood. That receptor grabs IgG inside the endosome, a small acidic bubble within the cell, carries it to the other side and lets it go there, where the pH is neutral again. It is a revolving door that picks up its cargo only on the maternal side and releases it on the fetal side.
The timing matters. At 17 to 22 weeks the fetus has only 5% to 10% of what the mother has, and by term its level usually exceeds hers. The bulk of the transfer happens at the end.
And it has a consequence that comes as a surprise: those borrowed antibodies can inhibit the baby's response to some vaccines, though that effect varies a great deal between vaccines and between studies.
In brief
IgG is the only class of antibody that crosses the human placenta in quantity.
The neonatal Fc receptor, FcRn, in the syncytiotrophoblast carries it across: it traps IgG in the acidic endosome and releases it on the fetal side.
A revolving door: it grabs on the maternal side and releases on the fetal side.
At 17 to 22 weeks the fetus has 5% to 10% of maternal levels, and by term usually exceeds them.
Those borrowed antibodies can inhibit an infant's response to some vaccines, with a highly variable effect.
Sources (1)
- Clinical and Developmental Immunology, via PubMed CentralIgG Placental Transfer in Healthy and Pathological PregnanciesPublished
This page explains the topic in general terms; it does not tell anyone what treatment to follow.
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