Describe the structure of an antibody (immunoglobulin G).

IgG antibodies have a Y-shaped structure consisting of four polypeptide chains: two identical heavy chains (~50 kDa each) and two identical light chains (~25 kDa each), connected by disulfide bonds. Key regions include: Fab (Fragment antigen-binding) - contains variable regions (VH and VL) that form the antigen-binding site with CDRs (Complementarity Determining Regions) providing specificity. Fc (Fragment crystallizable) - constant region that interacts with immune cells and complement, determining effector functions. The hinge region provides flexibility between Fab and Fc. Glycosylation at Asn297 in CH2 domain is critical for Fc receptor binding and effector functions. Understanding structure guides antibody engineering for improved properties.