Page 530 - Veterinary Immunology, 10th Edition

assembly of a partner chain. In B cells this is the light chain. The
VetBooks.ir partner chain is the one that uses only V and J genes and thus

contributes much less diversity to the antigen receptor, although it
tends to “fine-tune” its antigen-binding abilities. Once a complete

heavy chain has been formed using V, D, and J genes, further
recombination and rearrangement of its genes are stopped thus
preventing assembly of the second heavy chain allele.

Potential Immunoglobulin Diversity

Gene rearrangement generates enormous V-region diversity and
antigenic specificity in several ways. In humans, for example, only
one of a possible 80 IGKV genes is selected for transcription, as is
only one of the five IGKJ genes. Random joining of these will

generate 400 (80 × 5) different light chain V regions. With 300 IGHV,
5 IGHD, and 2 IGHJ genes available, as many as 3000 (300 × 5 × 2)
different heavy chain V regions can be generated. Since paired

heavy and light chains are used to form the antigen-binding site,
the total number of possible combinations is 1.2 million (400 ×
3000). In addition, the presence of two splice sites multiplies the
potential for diversity generated as a result of base deletion and
insertion. However, as pointed out previously, many of the gene

combinations so formed may be of little functional use.
Taking all possible mechanisms into account, the number of
antigen-binding sites and hence binding specificities generated in
16
humans is about 1.8 × 10 without accounting for somatic mutation.
7
(This figure may be compared with the estimated 1 × 10 antigenic
determinants that the immune system may recognize.)

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