A human IgG1 molecule. Heavy (γ) chains are clear; light (κ or λ) chains are shaded. Individual domains (V, variable; C, constant) are depicted as rectangles. The paired C_γ2 and C_γ3 domains with their N-linked glycans (yellow) make up the Fcγ module. The remainder of the molecule (including its hinge) is composed of 2 Fab′γ modules. The hinge (γ-chain residues 221-237) has a rigid core with 2 γ-γ interchain disulfide bonds, whereas the upper and lower hinges are flexible. One of the 2 set of docking sites on Fcγ is shown. The site for all Fcγ receptors (red) involves the medial aspects of both C_γ2 domains, extends into the lower hinge, and has a floor formed by the 2 Fcγ glycans.¹⁰⁷  The site for FcRn (blue) involves both C_γ2 and C_γ3 domains at their interface.¹⁰⁹  The site for complement C1q (green) is on the surface of the C_γ2 domain lateral to the Fcγ receptor site.¹¹⁰  The contralateral set of sites is obscured. In the FabIgG derivative,¹⁰⁸  the hinge disulfide bonds are cleaved; and a third Fab′γ module, with antibody specificity for the activating FcγRIIIA, is attached to one of the resulting cysteine residues. The original FcγR site is deliberately disabled so that there is little residual affinity for FcγRIIB. The sites for FcRn and complement C1q are little affected.