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In page Antibody:

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Following activation with antigen, B cells begin to proliferate rapidly. In these rapidly dividing cells, the genes encoding the variable domains of the heavy and light chains undergo a high rate of point mutation, by a process called somatic hypermutation (SHM). SHM results in approximately one nucleotide change per variable gene, per cell division.[1] As a consequence, any daughter B cells will acquire slight amino acid differences in the variable domains of their antibody chains.[citation needed]