Immunoglobulin G models.

(a). Graphical model of native (closed) IgG molecule consisting of two heavy chains of γ type and two light chains of either κ or λ type linked by disulphide bridges. The variable part of a heavy chain domain together with the variable part of a light chain domain (VL) together form the antigen binding site. A light chain together with variable heavy and CH1 domains form a Fab (fragment antigen-binding) part (“arm”), and the CH2 and CH3 domains constitute the Fc (fragment constant or crystallizable) part with effector functions. RF binding sites and effector sites for C1/C1q and FcRs reside in the Fc (CH2-CH3) domains and are shielded by the Fab “arms”. (b). Model of the conformational change in IgG from closed to open upon antigen binding. Epitopes for RFs, C1q, FcRs and protein A/G are indicated on the Fc. CL: constant light, VL: variable light, VH: variable heavy, CH: constant heavy, Ag: antigen, RF: rheumatoid factor, FcR: Fc receptor.