An immunoglobulin molecule is composed of four chains: two identical light chains, shown on the inner aspects of the molecule (in blue and gray), and two identical heavy chains, shown along the outer aspect of the molecule (in red and tan). Each chain is made up of immunoglobulin domains, represented as circles, and contains an internal disulfide bond (not shown). There are usually multiple disulfide bonds linking the two H chains in the hinge region and also a disulfide linking the H and L chains. Only IgG, IgA, and IgD have distinct hinge regions, and only IgM and IgE have four H chain domains. The points of cleavage of the enzymes pepsin and papain are shown. The CS is formed by structures contributed by the variable regions of both the heavy chain (VH) and light chain (VL).
VL: variable region of the light chain; CS: combining site; CL: constant region of the light chain; VH: variable region of the heavy chain; CH: constant region of the heavy chain; Ig: immunoglobulin.
