Page 545 - Review of Medical Microbiology and Immunology ( PDFDrive )

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Cell-Mediated Immunity 61
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CHAPTER C ONTENT S

Introduction
Cell-Mediated Reactivity
Tests for Evaluation of Cell-Mediated Immunity
Self-Assessment Questions
In Vivo Tests for Lymphoid Cell Competence (Skin Tests)
Practice Questions: USMLE & Course Examinations
In Vitro Tests for Lymphoid Cell Competence

INTRODUCTION against other microorganisms. In particular, measles virus
infection in people infected with M. tuberculosis can result in
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Although humoral (antibody-mediated) immunity is an
a loss of purified protein derivative (PPD) skin test reactiv-
important host defense against many bacterial and viral dis-
ity, reactivation of dormant organisms, and clinical disease.
eases, in many other bacterial infections (especially intracel-
lular infections such as tuberculosis) and viral infections, it is
measles virus binds to its receptor on the surface of human
primarily the cell-mediated arm that imparts resistance and
macrophages, the production of interleukin-12 (IL-12) by
aids in recovery. Furthermore, cell-mediated immunity is
the macrophages, which is necessary for cell-mediated
important in defense against fungi, parasites, and cancers. It
immunity to occur, is suppressed.
is also the main mechanism involved in the rejection of
The terms primary and secondary response are associated
organ transplants. The strongest evidence for the importance
primarily with antibody formation as described in Chapter
of cell-mediated immunity comes from clinical situations in
which its suppression (by immunosuppressive drugs or dis-
pattern. After the initial exposure to the antigen, the specific
ease, e.g., acquired immunodeficiency syndrome [AIDS])
T cell proliferates to form a small clone of cells (i.e., a primary
results in overwhelming infections or tumors. 60, but the timing of the T-cell response also follows the same
response occurs). Then, on subsequent exposure to the anti-
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The constituents of the cell-mediated immune system
gen, the small clone expands, and many more specific T cells
include several cell types: (1) macrophages, which present
are formed. These cells constitute the secondary response.
the antigen to T cells; (2) helper T cells, which participate
Although the interactions between various cells and
in antigen recognition and in regulation (helper and sup-
pressor) functions (see Chapter 58); (3) natural killer (NK)
ple: In the person with competent cellular immunity,
cells, which can inactivate pathogens; and (4) cytotoxic
opportunistic pathogens rarely or never cause disease, and
T cells, which can kill virus-infected cells with or without
the spread of other agents—for example, certain viruses
antibody. Macrophages and helper T cells produce cyto-
(e.g., herpesviruses) or tumors (e.g., Kaposi’s sarcoma)—is
kines that activate helper and cytotoxic T cells, leading to
limited. The assessment of the competence of cell-mediated
the killing of the pathogen or tumor cell.
An excellent example of cell-mediated immunity is the
delayed hypersensitivity response to the tuberculin skin
TESTS FOR EVALUATION OF
test in people who have been infected with Mycobacterium immunity is therefore important.
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tuberculosis. An area of skin induration occurs at the site of
CELL-MEDIATED IMMUNITY
the PPD injection subcutaneously. The induration is caused
by the presence of macrophages and helper T cell that
Evaluation of the immunocompetence of persons depends
respond to the purified protein derivative (PPD) extracted
from M. tuberculosis.
ity to commonly present antigens (equating the ability to
Infection with some viruses, namely, measles virus and
respond with the competence of cell-mediated immunity)
cytomegalovirus, can suppress cell-mediated immunity
or on laboratory assessments of T cells.
534
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