Immunoglobulin G (IgG) is the only antibody isotype that can cross the human placenta. This transfer occurs via neonatal Fc receptors (FcRn) on the syncytiotrophoblast cells of the placenta. This process is crucial for providing the developing fetus and newborn with passive immunity against various pathogens during the first few months of life before the infant's own immune system fully matures.

Antibodies, also known as immunoglobulins, are complex glycoproteins. Their structure is primarily composed of polypeptide chains folded into specific shapes to recognize and bind to antigens. They are synthesized by plasma cells as part of the adaptive immune response.

A standard immunoglobulin (antibody) molecule, such as IgG, is composed of four polypeptide chains: two identical heavy chains and two identical light chains. These chains are held together by disulfide bonds to form a characteristic Y-shaped structure. The variable regions at the tips of the arms are responsible for antigen binding, while the constant regions determine the antibody's effector functions.