The primary immunoglobulin present at birth is immunoglobulin G (IgG).
Human infants receive a significant portion of their early immunity from their mothers. This crucial protection is primarily delivered through the transfer of antibodies across the placenta during pregnancy. As the provided reference states, infants receive the majority of maternal immunoglobulins (Ig), predominantly immunoglobulin G (IgG), via the placenta.
This maternal antibody transfer begins relatively early in gestation, starting around the 13th week. As pregnancy progresses, the concentration of maternal IgG in the fetal circulation increases, providing the newborn with passive immunity against various pathogens the mother has encountered.
Understanding Maternal IgG Transfer
Why is IgG the main immunoglobulin transferred?
- Unique Structure: IgG is the only antibody class capable of crossing the placenta. This ability is due to specific receptors on placental cells that bind to the Fc region of IgG molecules, facilitating their transport from the maternal circulation into the fetal blood.
- Broad Protection: Maternal IgG provides the newborn with passive immunity against a wide range of bacteria, viruses, and toxins to which the mother has developed immunity. This protection is vital during the first few months of life before the infant's own immune system is fully mature and capable of producing significant amounts of antibodies.
Key Points About Immunoglobulins at Birth
Here are some key takeaways regarding immunoglobulins in newborns:
- Predominant Type: IgG is the main immunoglobulin found in significant quantities in a newborn's blood at birth.
- Source: This IgG is maternal in origin, transferred passively across the placenta.
- Timing: Transfer starts early in pregnancy (around 13 weeks) and increases significantly in the third trimester.
- Function: Provides crucial passive immunity, protecting the infant from infections in the early postnatal period.
- Other Immunoglobulins: While IgG is predominant, newborns do synthesize small amounts of other antibodies like IgM and IgA in utero if exposed to infection. However, their primary protection at birth comes from maternal IgG.
This maternal IgG gradually declines over the first few months of life as the infant's own immune system begins to produce its own antibodies.
