The linker region of Immunoglobulin D (IgD), the longest hinge sequence of any immunoglobulin, connects the Fab and Fc fragments. It has up to seven O-linked oligosaccharide sites, which is thought to be related to the flexible T-shape of IgD [1].

1: Semi-extended solution structure of human myeloma immunoglobulin D determined by constrained X-ray scattering. Sun Z, Almogren A, Furtado PB, Chowdhury B, Kerr MA, Perkins SJ; J Mol Biol. 2005;353:155-173. PMID:16157351