What is the only antibody that is able to cross the placenta?

The antibody that is able to cross the placenta is IgG. This property is significant because it allows for the transfer of maternal antibodies to the fetus, providing passive immunity during the critical periods of development and shortly after birth.

IgG is the most abundant immunoglobulin in human serum and is structured in a way that enables it to navigate through the placental barrier effectively. This transfer occurs mainly during the third trimester of pregnancy, which is vital for protecting the newborn from infections after birth until their immune system matures and begins producing its own antibodies.

In contrast, IgA primarily functions in mucosal immunity and is not designed to cross the placenta. IgM is typically involved in the primary immune response, and its large size prevents it from passing through the placenta. IgE is associated with allergic reactions and parasitic infections and also does not have the capability to cross the placental barrier. Therefore, IgG's unique ability to cross the placenta is fundamental for fetal protection.