Genetic diversity in immunoglobulin and T-cell receptor genes

The total diversity, a bout 1018 possible combinations
for all types of genes for immunoglobulins
and T-cell receptors, is the result of
different mechanisms. To begin with, different
numbers of variable DNA segments are available
for different chains (250–1000 for the H
chain, 250 for the L chains, 75 for the ! chain of
the T-cell receptor TCR!, etc.). The different D
and J segments also multiply the number of
possible combinations. Finally, DNA sequence
changes (somatic mutations) occur regularly in
the hypervariable regions, further increasing
the total number of possible combinations.