Variable and constant regions

Contrast of the amino acid series of several immunoglobulin   polypeptides has shown in which each light chain has a variable region at its N-terminal end and an invariant or constant region at its C terminus. Similarly every heavy chain has an N-terminal variable region and a C-terminal constant region. Because  it  is  the  N-terminal  component  of  the  light  and  heavy  chains  which  form  the antigen-binding   site  the  variability  in  amino  acid  series  of  these  regions explains  how hard  sites with various  specificities  for antigen  binding  can be established. In real, the variability in the variable regions of both heavy and light chains is commonly localized to three hypervariable regions in each chain. In the 3D structure  of the immunoglobulin  molecule,  the hyper- variable  component  of  the  light  and  heavy  chains  are  looped  together  to  build  the antigen-binding  site. The remaining  parts  of every variable  region  stay reasonably constant  in sequence,  generally  do not contact  the antigen  directly  and are called framework regions.

Figure:   Structure of an antibody molecule. (a) Each antibody molecule consists of two identical light chains and two identical heavy chains.

The molecule has two antigen-binding sites, each formed by a light chain and a heavy chain. (b) The N-terminal regions of the light and heavy chains are variable in amino acid sequence from antibody to antibody (variable regions; V regions) whilst the C-terminal regions are relatively constant in sequence (constant regions; C regions). The generic terms for these regions in the light chain are VL and CL and for the heavy chains are VH and CH.