What class of antibodies (immunoglobulins) can pass from the mother to the fetus through the placenta?

From the Guidelines

Immunoglobulin G (IgG) is the only class of antibodies that can pass from the mother to the fetus through the placenta. This transfer occurs via an active transport process involving Fc receptors on placental cells, primarily during the third trimester of pregnancy, as described in a study published in the American Journal of Obstetrics and Gynecology in 2022 1. The placental transfer of maternal IgG provides the newborn with passive immunity against various pathogens for the first few months of life, which is crucial since the infant's immune system is still developing after birth. This passive immunity helps protect the newborn against infections until their own immune system matures enough to produce adequate amounts of antibodies. The other classes of immunoglobulins (IgA, IgM, IgD, and IgE) are too large to cross the placental barrier efficiently. Of the IgG subclasses, IgG1 and IgG3 cross most efficiently, while IgG2 and IgG4 transfer less effectively, as supported by a study published in Gut in 2019 1.

Some key points to consider:

• The transfer of IgG antibodies from mother to fetus is an active process that involves the binding of IgG to Fc receptors on placental cells.
• This process is crucial for providing the newborn with passive immunity against various pathogens.
• The IgG subclasses have different efficiencies in crossing the placental barrier, with IgG1 and IgG3 being the most efficient.
• The other classes of immunoglobulins (IgA, IgM, IgD, and IgE) do not cross the placental barrier efficiently due to their larger size.

It's worth noting that while other studies have discussed the transfer of antibodies from mother to fetus, the most recent and highest quality study, published in 2022 1, confirms that IgG is the only class of antibodies that can pass through the placenta. Therefore, IgG is the primary class of antibodies responsible for providing passive immunity to the newborn.

From the Research

Antibody Class Transfer

• The class of antibodies that can pass from the mother to the fetus through the placenta is IgG [ 2, 3, 4, 5, 6 ].

Characteristics of IgG Transfer

• IgG is the only antibody class that significantly crosses the human placenta [ 2 ].
• The transfer of IgG depends on several factors, including maternal levels of total IgG and specific antibodies, gestational age, placental integrity, IgG subclass, and the nature of the antigen [ 2 ].
• The neonatal Fc receptor (FcRn) mediates the transfer of IgG across the placenta [ 2, 4, 6 ].

IgG Subclasses and Transfer

• IgG1 is the most efficiently transported subclass, while IgG2 is the least [ 6 ].
• Other IgG subclasses, such as IgG3, can also be transported across the placenta, but their efficiency may vary [ 4 ].

Clinical Implications

• The transfer of IgG from mother to fetus provides protection to the infant against infections until the infant's humoral response is efficient [ 2, 3 ].
• Maternal immunization strategies can take advantage of this transfer to protect newborns against neonatal and infantile infectious diseases [ 2 ].