||P.113 bottom paragraph to p.114 2nd paragraph: "3-2 Immunoglobulin heavy and light chains are composed of constant and variable regions: The amino acid sequences of many immunoglobulin heavy and light chains have been determined and reveal two important features of antibody molecules. First, each chain consists of a series of similar, although not identical, sequences, each about 110 amino acids long. Each of these repeats corresponds to a discreet, compactly folded region of protein structure known as a protein domain. The light chain is made up of two such immunoglobulin domains, whereas the heavy chain of the IgG antibody contains four (see Fig. 3.1a). This suggests that the immunoglobulin chains have evolved by repeated duplication of an ancestral gene corresponding to a single domain.
The second important feature revealed by comparisons of amino acid sequences is that the amino-terminal sequences of both the heavy and light chains vary greatly between different antibodies. The variability in sequence is limited to approximately the first 110 amino acids, corresponding to the first domain, whereas the remaining domains are constant between immunoglobulin chains of the same isotype." Wikipedia 'Antibody' heading 'Immunoglobulin domains': "The Ig monomer is a "Y"-shaped molecule that consists of four polypeptide chains two identical heavy chains and two identical light chains connected by disulfide bonds [ref 13]. Each chain is composed of structural domains called Ig domains."