Which immunoglobulin class is most likely to cross the placenta?

The immunoglobulin class that is most likely to cross the placenta is IgG. This ability is significant because IgG is the main type of antibody found in the blood and the extracellular fluid, and it plays a crucial role in providing immunity to the fetus during pregnancy. As the pregnancy progresses, maternal IgG antibodies can transfer through the placenta into the fetal circulation, offering passive immunity to the newborn. This transfer significantly helps protect the infant from infections during the initial months of life, as the newborn's own immune system is still developing.

IgA, while important for mucosal immunity and found in secretions such as breast milk, does not cross the placenta effectively. IgD is primarily found on the surface of B cells and does not play a significant role in immunity throughout the placenta. IgM, although it is the first antibody produced in response to infection and a primary response, is too large to effectively cross the placenta. Therefore, IgG is the only immunoglobulin that can efficiently traverse the placental barrier to provide the necessary immune support to the developing fetus.