Which class of antibodies is capable of crossing the placental barrier?

The class of antibodies capable of crossing the placental barrier is immunoglobulin G, commonly known as IgG. This ability is crucial for providing passive immunity to the fetus during pregnancy. IgG antibodies are produced by the mother in response to infection or vaccination and can transfer through the placenta to the developing fetus, offering protection against pathogens during the early stages of life, before the infant's immune system becomes fully developed.

On a molecular level, IgG has a specific structure that allows it to bind to receptors on placenta cells, facilitating its transport from maternal to fetal circulation. This transfer is particularly important as it helps to shield the fetus from infections and contributes to the establishment of immunity in the newborn after birth.

The other classes of antibodies, such as IgA, IgM, and IgE, do not cross the placenta effectively. IgA primarily functions in mucosal areas and is found in secretions like breast milk, helping protect infants after birth. IgM is usually the first antibody produced in response to an infection but is too large to cross the placental barrier. IgE is involved in allergic reactions and defense against parasitic infections, but it also does not have the capability to pass through the placenta.