A Fc receptor mediates the transport of IgG across the barrier separating the maternal and fetal blood stream: Article IgG Placental Transfer in Healthy and Pathological Pregnancies

"Placental transfer of maternal IgG antibodies to the fetus is an important mechanism that provides protection to the infant while his/her humoral response is inefficient. IgG is the only antibody class that significantly crosses the human placenta. This crossing is mediated by FcRn expressed on syncytiotrophoblast cells."

Hello Aqsa Hussain

IgG is the main immunoglobulin in the blood system, accounting for approximately 80% of the total circulating immunoglobulin. It is also present in the tissue spaces. IgG are usually of higher affinity and can neutralize toxins, viruses, and bacteria, opsonize them for phagocytosis, and activate the classical pathway of the complement system.

It is important to protect infants during the first months of life against infections. Therefore, IgG is the only class of immunoglobulin capable of crossing the placenta thereby providing some degree of immune protection to the developing fetus.

IgG is a large protein with a tetrameric structure, containing two heavy and two light chains disposed in a Y-like shape. In its structure, there is an antigen binding site (Fab region) and a constant region, the crystallizable fragment (Fc), which binds to Fc receptors found on the surface of different cells in the immune system, most importantly on phagocytes.

All the four types of IgG (1 to 4) are transferred across the placenta via syncytiotrophoblast cells that express receptor for the Fc domain, called the neonatal Fc receptors (FcRn) providing some degree of immune protection to the developing fetus and the infant until the infant’s immune system can produce its own antibodies.

Best.