Evolution of genes of the immunoglobulin supergene family

Distinct evolutionary relationships can be recognized
in the homology of genes for Ig-like
molecules. A precursor gene for a variant (V)
and a constant (C) region must have arisen by
duplication and subsequent diversification of a
gene for a primordial cell surface receptor. Such
a primordial gene could have looked like the
gene for thymosine (Thy-1) or poly-Ig receptor.
No somatic recombination occurs in these gene
families or in the genes of the MHC complex.
In contrast, the rearrangement of lymphocyte
germ-line genes by somatic recombination
during the maturation of B cells and T cells is
the basis for the formation of immunoglobulins,
T-cell receptors, and CD8. Somatic recombination
of the genes for antigen-binding molecules
was an enormous evolutionary advantage. Consequently,
this is found even in early vertebrates.