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Molecular Immunology
methods used in research and in monitoring the use of
prescribed and illegal drugs. Monoclonal antibodies are
used in highly sensitive and specific immunoassays. In
many immunochemical assays monoclonal antibodies are
preferred to polyclonal antibodies because of their homo-
geneity, single-epitope specificity, and consistent affinity.
Monoclonal antibodies have made possible a very use-
ful, convenient, and highly specific type of immunoas-
say. This assay uses a monoclonal antibody to one epitope
on the antigen to capture and bind the antigen. The cap-
tured antigen is then reacted with a second monoclonal (or
polyclonal) antibody that is specific to the antigen and to
which an enzyme has been covalently linked. After wash-
ing away the excess antibody and its linked enzyme, the
amount of the antigen can be determined by measuring
the activity of the enzyme that is bound to the second an-
tibody. Such assays are designated “sandwich” enzyme-
linked immunoassay (ELISA).
Immunoassays that detect viral antigens, e.g., various
hepatitis viruses, lymphotropic viruses, and HIV, are used
to exclude blood from donors who might be capable of
transmitting disease because of a prior infection. The pres-
ence of antibodies to viral antigens can also determined by
immunassay methods to ascertain if an individual might
have been previously infected with the virus when the
presence of the virus itself cannot be detected.
Monoclonal antibodies linked to therapeutic drugs can
be used to target malignant cells for site-specific drug de-
livery. Monoclonal antibodies are also used to block cell
receptors when interaction of the receptor with its natu-
ral ligand causes undesirable effects or disease. A mono-
clonal antibody called abciximab is specific for the platelet
fibrinogen receptor (GpIIb/IIIa) and is administered after
angioplasty and other cardiovascular surgical procedures
to prevent platelet (hemostatic) plug formation at the sur-
gical (injury) sites. However, mouse monoclonal antibod-
ies are foreign molecules and are themselves immuno-
genic. To overcome this problem, monoclonal antibodies
are “humanized” by replacing the mouse antibody genes
for the constant region in the hybridoma with human an-
tibody genes. Therapeutic antibodies such as abciximab
have been “humanized” so that they can be used therapeu-
tically with minimal adverse effects.
Recognition of Infected Cells by Cell Receptors
Recognition of the antigen by an antibody is the re-
sult of complementarity between the antigen and specific
amino acid residues in the hypervariable regions of the im-
munoglobulin light and heavy chains. Some cell receptors
(Figure 35-1) are immunoglobulin molecules, e.g., IgM,
IgD, and IgA. Humoral immunity, antibody-dependent,
cell-mediated cytotoxicity (Figure 35-4), macrophage
attack on foreign cells via Fc receptor recognition of
antibody-tagged cells (Figure 35-2), and complement-
based cell membrane attack incorporate this antigen-
antibody recognition mechanism for identifying and re-
acting to foreign substances and cells.
Acquired cell-mediated immunity involves complex in-
teractions between antigen molecules and immune cells
that recognize the antigens. Finely tuned, cell-based mech-
anisms are paramount in mounting responses to foreign or-
ganisms. T cells (lymphocytes) are the key players in this
response. There are two principal types of T cells that are
identified by the presence of either CD4+ or CD
+ core-
ceptor proteins on the T-cells surface. CD4+ T cells are
T-helper (Th) cells. These cells recognize peptides from
foreign proteins that are processed and presented on the
infected cell membrane by MHC class II proteins. CD
T cells recognize peptides from foreign proteins that are
presented on the cell membrane by MHC class I molecules
(see Figures 35-1 and 35-3).
Attack on infected cells by cytotoxic CD
+ T cells
(CTLs or Tc cells) requires that the peptides presented
by the MHC I protein on the surface of the host cell be
recognized as nonself. The antigen-presenting cell (APC)
is thus marked as a target for destruction (Figure 35-3).
The MHC I surface complex that culminates in destruction
of virus-infected cell involves foreign peptides that have
been synthesized by the infected cell and subsequently
transported to the cell surface MHC I protein. Peptides
from infecting bacteria may be synthesized by the bacte-
ria themselves but are still presented by MHC I to mark the
cell for destruction by the CTLs. The processing and trans-
port of newly synthesized MHC I proteins and the foreign
peptides through the endoplamic reticulum to the APC
membrane involves several proteins. These additional pro-
teins “usher” the MHC I proteins and the peptides until the
MHC I proteins and peptides meet and then release them
to the Golgi apparatus for final movement to the exterior
of the APC. Arrival of an MHC I protein with its bound
peptide is followed by binding of a CTL to the APC via
a TcR specific for the forign peptide on the CD
+ CTL.
Action by the CTL is aided by the CD3 molecule, which
acts as a cofactor to the TcR.
Discrimination between nonself-derived peptides, i.e.,
those from a pathogen, and self-derived peptides requires
that the chemical interactions between the T cell via its
TcR and the MHC protein with its bound peptides be ex-
tremely specific. This specificity is a property of the par-
ticular TcR of the individual T-cell clone.
The MHC I protein contains two polypeptide chains;
chain contains two distinct domains, a 1 and
that are on the outside surface of the antigen-presenting
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