Human IgG is a complex molecule composed of four ... Each heavy chain is attached to a light chain by disulfide bonds, and the two heavy chains are likewise attached by a third disulfide bond.

Structural variations, primarily in the hinge region (size and flexibility) and the number and position of disulfide bonds, lead to four distinct IgG subclasses: IgG1, IgG2, IgG3, and IgG4 ...