Basic immunoglobulin structure and IgG1 allotypes. (a). Basic immunoglobulin structure. CH1, 2 and 3 are the constant heavy chains. CL is the constant light chain. VH is the variable heavy chain and VL the variable light chain which together form the variable domain of the immunoglobulin, a specific antigen binding site, also referred to as the idiotype. (b). IgG1 allotypes . The white residues in the constant parts are those residues which differ by allotype in human IgG1. There is a Lys (G1m17) for Arg (G1m3) change at codon 214 in the CH1 domain, an Asp 356 Leu 358 (G1m1) for Glu 356 Met 358 (nG1m1) in the CH3 domain and a Gly 431 (G1m2) for Ala (nG1m2) also in the CH3 domain. The nG1m1 and nG1m2 are not "true" allotypes because these amino acid residues are present in other IgG subclasses and are not expected to be immunogenic in the individual.