All five immunoglobulin classes share the same basic four polypeptide chain structure of two heavy-chains and two light chains. There are five heavy chain types，and two light-chain types (Kappa and Lambda) both having a molecular weight of 22.5kDa. Any heavy-chain type can associate with either light-chain type，but on any immunoglobulin molecule both light-chains are of the same type. Kappa and Lambda consist of a variable region and a constant region and can easily be differentiated by the antigenic properties of the constant region.