If each of the many different immunoglobulin
molecules with their variable regions were
coded for by separate genes, many millions of
genes would be required. This is not the case.
Rather, during lymphocyte differentiation, a
practically unlimited number of different cells
are produced by recombination of a large but
limited number of genes. This occurs by somatic
recombination of genes during the differentiation
of B cells and T cells. Antibody diversity
arises by the following genetic mechanisms:
(1) multiple DNA sequences of the germline
genes for H and L chains code for Ig
molecules with different specificities; (2) somatic
recombination of the various DNA segments
greatly increases the number of possible
combinations; (3) somatic mutations occur in
the hypervariable regions, leading to further
genetic differences.
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