This is a foundational, textbook fact of immunology describing how the adaptive immune system generates antibody diversity and affinity maturation, so no citation is needed.
The claim states that antibody diversity and antigen binding site variation are generated by V(D)J recombination and somatic hypermutation. This is a well-established, fundamental biological fact of adaptive immunity found in every immunology textbook. Per the instructions, statements that are trivially true by direct everyday observation or established biological definition are classified as COMMON_KNOWLEDGE, requiring no citation or paper assignment.