Antibody domains

Every light chain  consists  of two repeating  segments  of about  110 amino  acids which  fold  into  two  compact  3D domains,  one  representing  the variable region of the light chain and the other domain representing the constant region.  Every heavy chain is also made up of repeating units about 110 amino acids long. Because every immunoglobulin G heavy chain is about 440 amino acids large it forms four  domains,  one  domain  for  the  variable  region  and  three  domains  in  the constant  region.  The similarity of amino acid sequence among the several domains suggests which they arose in evolution through gene duplication.