One unambiguous message that comes from the data obtained relates to the central role played by the hinge region, and its constituent disulfide bonds, in the functioning of the IgG molecule. Segmental flexibility allows variation in the distance between the antibody combining sites, located at the distal ends of the Fab regions, and serves the important function of allowing bivalent attachment of antibodies to antigenic determinants even if the latter form a fixed array, for example, on a cell surface (monogomous bivalency). Too much flexibility, however, seen as a consequence of cleaving the hinge-region disulfides appears to be inimicable to the expression of effector functions. The hinge region and its disulfides control the degree of flexibility, thus allowing the IgG molecule to perform its varied functions.