Which of the follwoing molecular phenomenon in Ig genes is responsible for affinity maturation of antibody response

C i.e. Somatic Hypermutation Affinity Maturation refers to the increased affinity of antibodies for their specific antigen as the antibody response continues. This phenomenon is not seen in IgM responses. Those B-cells expressing receptors with a higher affinity for the antigen are selctively expanded, even as the levels of antigen fall. Thus B-cells with higher affinity are generated during the 1deg & 2deg immune respones in large pa due to Somatic hypermutations that occur in VI I genesQ. (VH =Variable region of Heavy chain).