Immunoglobulin G (IgG) is the predominant antibody isotype in human serum and extracellular fluid, accounting for approximately 75% of circulating immunoglobulins. It is a large Y-shaped glycoprotein with a molecular weight of about 150 kDa, composed of two identical heavy gamma chains and two identical light chains connected by disulfide bonds and non-covalent interactions. IgG functions in humoral immunity by binding antigens via its Fab regions, facilitating neutralization of viruses and toxins, opsonization for phagocytosis, and activation of the classical complement pathway through its Fc region. It is the only antibody capable of crossing the placenta, providing passive immunity to the fetus. In food science contexts, IgG is abundant in bovine colostrum and milk, contributing to intestinal immunity. As a protein, it denatures at elevated temperatures and has no fixed boiling or melting point.