Clinical Biochemistry of Domestic Animals (Sixth Edition) - UMK ...

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Chapter | 5 Proteins, Proteomics, and the Dysproteinemias
the immunoglobulins observed in animals, IgA, IgM, and
IgE are found primarily in the γ 1 region (and to some
extent in the β 2 region), and IgG is found primarily in the
γ 2 region. The specific identification and quantification of
the immunoglobulins require the use of immunochemical
techniques (Section V.D.1). A brief description of the
immunoglobulins is given here as a basis for understanding
the interpretation of dysproteinemias visualized on SPE.
More thorough coverage is given in Chapter 6 on clinical
immunology.
1 . Source of Immunoglobulins
Immunoglobulins act as antibodies and are produced in
response to antigens. They are highly specific if only one
antigenic determinant is involved. In nature, however,
multiple antigenic determinants are usually involved. The
lymphocytic cell line plays the central role in the immune
system. There are two subpopulations, the B lymphocytes
(bursa) and the T lymphocytes (thymus), which can be
identified by special immunological means. The T cells are
found in blood and in lymph nodes in the deep cortical areas
and paracortical sinuses. They are associated with cell-mediated
immunity. The B cells were originally identified in the
bursa of Fabricius of the chicken. In the adult, they are found
in the blood and in the germinal centers of lymph nodes. The
B cells respond to antigenic stimuli with the proliferation of
plasma cells that produce the specific antibody against the
stimulating antigen. Five immunoglobulin classes, IgG, IgA,
IgM, IgD, and IgE, have been identified in humans but in
most domestic animal species only IgG, IgA, IgM, and IgE
are evident, although IgD may be rarely expressed ( Gorman
and Halliwell, 1989b ) .
A specific plasma cell population of defined genetic
origin—a clone—produces a specific immunoglobulin.
Uncontrolled growth of a single B-cell clone (malignancy)
results in the overproduction of a single chemical species of
immunoglobulin, which appears as a sharp “ monoclonal ”
spike or monoclonal gammopathy on an electrophoretogram.
Occasionally, a “biclonal ” or “triclonal ” gammopathy
can be identified. A group of clones, each of a different
genetic origin, can also overproduce a heterogeneous mixture
of immunoglobulins, which appears as a diffuse or
broad hyperglobulinemic region on the electrophoretogram.
This region is described as a “ polyclonal ” gammopathy.
2 . Structure of Immunoglobulins
The immunoglobulins are glycoproteins whose basic structure
is comprised of two heavy (H) and two light (L) chains
linked by disulphide bridges with a molecular mass of
150 kDa. The structure of the H chain governs the class of
immunoglobulin and is named by corresponding Greek letters:
γ in IgG, μ in IgM, α in IgA, ε in IgE, and δ in IgD.
The structure of the L chain is either kappa ( κ ) or lambda ( λ )
and denotes type. Structural variations in the variable
regions of H or L chains provide a basis for further subdivision
into subtypes and subclasses. To date, four subclasses
of IgG have been identified in humans (IgG1, IgG2, IgG3,
and IgG4), with two identified in cattle (IgG1 and IgG2),
four in dogs (IgG1, IgG2a, IgG2b, and IgG2c), and three in
horses (IgGa, IgGb, and IgGc). The horse also has IgG(T).
This was originally recognized in horses used to produce
antitetanus toxoid, and although it has a higher mobility than
γ -globulins, its amino acid sequence showed it was more
closely related to IgG ( Gorman and Halliwell, 1989b ).
IgG, IgD, and IgE are monomers; IgA is a dimer; and
IgM is a pentamer. Most viral, bacterial, and toxin antibodies
are of the IgG type and are present in all animals. IgE
is involved in allergic and anaphylactic reactions, whereas
IgA is found in the secretions of the respiratory, genitourinary,
and gastrointestinal tracts. IgA is a dimer of two basic
units joined by a secretory piece. IgM is a cyclic pentamer
of five basic units that forms a high-molecular-weight unit.
These are the macroglobulins or “ M ” components.
The Bence-Jones proteins are light-chain units, and their
presence reflects the asynchronous synthesis of H chains
so that excess L chains appear and are secreted in urine.
They are not detected on SPE, but they can be detected by
immunochemical techniques and are often found to accompany
gammopathies (Solomon, 1976) , especially multiple
myelomas.
E . Lipoproteins
A significant amount of protein in serum is associated with
lipid in the form of lipoproteins. These microscopic particles
are composed of a lipid core containing hydrophobic
molecules such as triglyceride and cholesterol ester with
a phospholipid monolayer on the surface. The apolipoproteins
are proteins that help to stabilize the structure
of the lipoproteins and also have biological activity
related to their function of transporting the immiscible
lipids through the aqueous environment of the circulation.
Lipoproteins can be characterized according to
their electrophoretic mobility ( α -, β -lipoprotein) but are
now mainly classified by their density as determined
by ultracentrifugation. The main classes are very low
density lipoprotein (VLDL), low-density lipoprotein
(LDL), and high-density lipoprotein (HDL). These
are involved with lipid transport between the liver and
peripheral tissues. There is a further class of lipoprotein,
the chylomicrons, which are involved in the transport of
dietary lipid from the intestine. Lipoproteins are the subject
of extensive research in human medicine, especially
in relation to diagnosis of diseases of the cardiovascular
system, but in domestic animals their analysis is not a
frequently requested test. The lipoprotein profile in
horses and dogs has been established ( Watson, 1996 ;
Watson and Love, 1994 ) and can be altered in disease,