Page 502 - Veterinary Immunology, 10th Edition

consequences are probably not great, especially since IgD has not
VetBooks.ir been identified and may not even be expressed in pigs. Pig IgE has

been identified. One IgM allotype has been reported (Box 16.2).

Box 16.2

Curious Case of the Camel

Members of the camel family from both the Old and New Worlds
(camels and llamas) have three IgG subclasses: IgG1, IgG2, and
IgG3. IgG1 has a conventional four-chain structure and therefore

has a molecular weight of 170 kDa. In contrast, IgG2 and IgG3,
which together account for 75% of camel immunoglobulins, are
100-kDa heavy-chain dimers that have no light chains! In addition,

camel IgG2 heavy chains lack a CH1 domain but compensate for
this by having a very long hinge region. Despite lacking light
chains, these molecules can still bind to many antigens. It has been
noted that these antibodies bind effectively to the substrate pockets
of enzymes. Studies have also shown that the antigen-binding site

on these heavy chains is very convex. This enables it to fit snugly
into the concave active site on an enzyme. Thus these single-chain
antibodies may have a structural advantage over conventional

immunoglobulins in neutralizing enzyme activity.

Dogs and Cats

Dogs have four IGHG genes and hence four IgG subclasses, named
IgG1, IgG2, IgG3, and IgG4, in order of abundance. In addition,

dogs have IgA, IgM, IgD, and IgE. Four alleles have been identified
in the dog IGHA gene. All are restricted to the hinge region. An IgM
allotype has been described in the dog.
Cats have two IGHG genes, one IgM subclass, and possibly two

IgA subclasses (IgA1 and IgA2), as well as two possible IgE
subclasses. The two IgG genes appear to be allotypes of one
subclass and have been designated IgG1a and IgG1b. They account
for about 98% of serum IgG. About 2% of cat serum

immunoglobulins belongs to a second uncharacterized IgG
subclass.

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