One example of such a basic folding pattern is the immunoglobulin domain, which got its name from the molecule on the right side of the picture: Immunoglobulin G. IGG2a consistst of two copies each of six different immunoglobulin domains arranged in two times two chains: The light chains consist of a variable domainVL and a constant domain CL, the heavy chains of the variable domain VH and the constant domains CH1, CH2 (glycosylated) and CH3. The chains are linked by disulfide bridges in the hinge region between CH1 and CH2.
Each of the immunoglobulin domain is a beta sandwich composed of two antiparallel beta sheets. The chain starts at the N-terminus contributing two strands to the front sheet, then switches to the back sheet and, three strands later, contributes two more strands to the front sheet, then the last two strands to the back sheet
Domains VL and VH together make up an FV fragment, which is responsible for antigen recognition. A Fab fragment consists of VL and CL, VH and CH1. The two chains are frequently linked by a disulfide bridge at the C-teminus of the two chains. A FC fragment consists of a dimer of two CH2-CH3 units. It represents the part of the molecule recognised by the FC receptor.