Biomedical Engineering Reference
In-Depth Information
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Antibodies that, by binding to the cell surface antigen, mark the tumour cell for destruction. NK
cells and macrophages express cell surface receptors that bind to the antibody F
c
region (Box 13.2).
Thus, antibody bound to tumour antigens directs these immune elements directly to tumour sur-
face. Antibodies also activate complement, which is capable of directly lysing tumour cells.
Box 13.2
Antibody architecture
Five major classes of antibodies (immunoglobulins, Igs) have been characterized: IgM, IgG,
IgA, IgD and IgE). Immunoglobulins of all classes display a similar basic four-chain structure
consisting of two identical light (L) chains and two identical heavy (H) chains (Figure 13.B2).
The overall structure is held together by disulfi de linkages and non-covalent interactions. Dif-
ferent H chain types are present in immunoglobulins of different classes. In addition, some
classes can be further subdivided into subclasses (isotypes) based upon more subtle differ-
ences. Thus, human IgG can be subdivided into IgG
1
, IgG
2
, IgG
3
and IgG
4
. Murine IgG can be
subdivided into IgG
1
, IgG
2a
, IgG
2b
and IgG
3
.
In their native conformations, each immunoglobulin chain is seen to be composed of discrete
domain structures, stabilized by intrachain disulfi de linkages (not shown below). Each domain
contains approximately 110 amino acid residues. H chains and L chains contain both variable (V)
and constant (C) domains. Variable regions house the actual antigen-binding site of the antibody.
Variable regions of antibodies displaying different (antigen-binding) specifi cities differ in amino
acid sequence. Constant regions (within any one antibody class/subclass) do not. L chains contain
one variable (V
L
) and one constant (C
L
) domain. H chains contain one variable (V
H
) and three
constant (C
H
1, C
H
2 and C
H
3) domains. In addition, H chains display a single short sequence join-
ing C
H
1 and C
H
2. This is the fl exible hinge (H) region, which contains several proline residues.
Treatment with certain proteolytic enzymes (e.g. papain) results in cleavage of the immu-
noglobulin at the hinge region, yielding two separate antigen-binding fragments (2
F
(ab)
), and a
constant fragment (F
c
). The F
c
region mediates the various antibody effector functions. F
ab
frag-
ments, although retaining their antigen-binding properties, are no longer capable of precipitating
antigen
in vitro
. However, immunoglobulin incubation with other proteases (e.g. pepsin) results
in antibody fragmentation immediately below the hinge region. This leaves intact two interchain
disulfi de linkages towards the C-terminus of the hinge region. This holds the two antigen-bind-
ing fragments together. The products of this fragmentation are donated F(ab)
2
and F
c
. Because of
its bivalent nature, F(ab)
2
retains the ability to precipitate antigen
in vitro
.
F
V
fragments consist of V
H
and V
L
domains, and can easily be produced by recombinant
DNA technology (as can other antibody fragments). Two F
V
domains can be stabilized by the
introduction of an interchain covalent linkage (e.g. a disulfi de linkage or via direct chemical
coupling). 'Single-chain' F
V
fragments may also be generated by the introduction of a short
peptide linker sequence between the two F
V
domains.
Selected regions within the antibody's variable domain display greater variability in amino
acid sequence (from one antibody to another) than do other variable regions. These so-called
'hypervariable' regions (complementarity-determining regions (CDRs)) are brought into close
proximity upon antibody folding into its native conformation, and represent the antigen binding
sites. The remaining areas of the variable domain are termed framework regions.
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