Problem:

Antibodies or immunoglobulins are proteins made by the immune system in response to alien(!) molecules. Each antibody binds to its specific antigen. This great diversity and specificity is cause of diversity in Antigen Binding Site of heavy chain and light chain of antibody. Although other sections of Antibody are highly stable, Binding sites are very mutant.

Question: What is the reason for this difference in mutation rates in different parts of antibodies? Please give all the reasons.