Page 498 - Veterinary Immunology, 10th Edition

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FIG. 16.15 The arrangement of the immunoglobulin heavy chain
genes of the major domestic species.

Horses

The horse heavy chain gene locus is located on chromosome 24qtr.

The locus contains seven IGHG genes, and all are expressed. Thus
there are seven IgG subclasses: IgG1 thru IgG7. (The previous
nomenclature for IgG1 thru IgG4 was IgGa, IgGc, IgG[T], and IgGb.
IgG6 was previously called IgG[B]). The order of the Ig heavy chain

genes in the horse is;
5′-M-D-G1-G2-G3-G7-G4-G6-G5-E-A-3′.
IgG7 is closely related to IgG4 and likely resulted from a recent
duplication of the IGHG4 gene. IgG1, IgG4, and IgG7 are produced

in response to intracellular infections, while IgG3 and IgG5 are
mainly produced in response to extracellular invaders. Horses also
express IgM, IgD, IgA, and IgE. The horse IGHD gene is located
downstream from IGHM. It appears to be expressed at least at the

b
a
mRNA level. Horses have two IgG4 alleles (IgG4 and IgG4 ) and
four IgE alleles (IgE ).
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