Page 493 - Veterinary Immunology, 10th Edition

VetBooks.ir Production of Immunoglobulin Heavy

Chains

Two genes code for each immunoglobulin heavy chain. One gene
codes for the variable domain (and thus the antigen-binding site),
whereas a separate gene encodes the constant domains. The way in
which genes code for the variable domains is discussed in Chapter

17. The genes that code for the constant regions of immunoglobulin
heavy chain (IGH genes) each consist of multiple expressed
sequences or exons. One exon codes for each constant domain, and
one codes for the hinge region (Fig. 16.12). A complete IgM constant

region gene (IGHM) therefore consists of five exons, whereas an
IgA constant region gene (IGHA) contains four exons. The heavy
chain constant region genes are clustered on one chromosome.
They are generally arranged in the order 5′-IGHM-IGHD-IGHG-

IGHE-IGHA-3′. Thus the gene for the µ chain is followed by the
gene for the δ chain, and these are followed by the γ chain genes
and so on.

FIG. 16.12 A peptide chain such as an immunoglobulin heavy
chain is coded for by a series of expressed sequences (exons)
separated by intervening sequences (introns). Usually each exon
codes for a single domain. When transcription occurs, the introns
are spliced out and the exon sequences joined together in RNA.

As they mature, B cells undergo two DNA recombination events.
The first, called V(D)J recombination, creates the antigen binding
site of the BCR while the B cells are developing within the bone

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