Page 502 - Review of Medical Microbiology and Immunology ( PDFDrive )

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mebooksfree.com mebooksfree.com mebooksfree.com virus-infected cells) CHAPTER 57 Immunity 491 mebooksfree.com
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TABLE 57–3 Major Functions of T Cells and B Cells

Antibody-Mediated Immunity (B Cells)
Cell-Mediated Immunity (T Cells)
1. Host defense against infection (opsonize bacteria, neutralize
1. Host defense against infection (especially M. tuberculosis, fungi, and
toxins and viruses)
2. Allergy (hypersensitivity) (e.g., hay fever, anaphylactic shock)
2. Allergy (hypersensitivity) (e.g., poison oak)
3. Graft and tumor rejection
3. Autoimmunity
4. Regulation of antibody response (help and suppression)
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mechanisms. The process by which these components
interact to cause inflammation is described in Chapter 8.
(i.e., a delayed hypersensitivity reaction specifically
Macrophages and certain other phagocytic cells such as
against M. tuberculosis).
Cytotoxic (cytolytic) T lymphocytes are also specific
dendritic cells participate in both the innate and adaptive
effectors of the cellular immune response, particularly
arms of the immune response. They are, in effect, a bridge
between the two arms. As part of the innate arm, they
against virus-infected cells. In this example, a virus (e.g.,
ingest and kill various microbes. They also present antigen
influenza virus) is inhaled and infects a cell of the respira-
to helper T cells, which is the essential first step in the acti-
tory tract. Viral envelope glycoproteins appear on the sur-
vation of the adaptive arm (see later). It is interesting to
note that neutrophils, which are also phagocytes and have
proteins. A cytotoxic T cell binds via its antigen-specific
excellent microbicidal abilities, do not present antigen to
receptor to the viral antigen–class I MHC protein complex
and is stimulated to grow into a clone of cells by interleu-
helper T cells and therefore function in innate but not face of the infected cell in association with class I MHC
kin-2 produced by helper T cells. These cytotoxic T cells
acquired immunity.
mebooksfree.com mebooksfree.com mebooksfree.com I MHC protein complexes on the cell surface and releasing mebooksfree.com
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specifically kill influenza virus–infected cells (and not cells
infected by other viruses) by recognizing viral antigen–class
SPECIFICITY OF THE IMMUNE
RESPONSE
perforins that destroy the membrane of the infected cell.
Cell-mediated immunity and antibody are both highly spe-
2. Antibody-Mediated Immunity
cific for the invading organism. How do these specific
protective mechanisms originate? The process by which
Antibody synthesis typically involves the cooperation of
these host defenses originate can be summarized by three
three cells: antigen-presenting cells (e.g., dendritic cells
actions: (1) the recognition of the foreign organism by
specific immune cells, (2) the activation of these immune
cessing by an antigen-presenting cell, fragments of antigen
cells to produce a specific response (e.g., antibodies), and
appear on the surface of that cell in association with class
(3) the response that specifically targets the organism for and macrophages), helper T cells, and B cells. After pro-
II MHC proteins. The antigen–class II MHC protein com-
destruction. The following examples briefly describe how
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plex binds to receptors on the surface of a helper T cell
specific immunity to microorganisms occurs. An overview
specific for that antigen. This activates the helper T cells to
of these processes with a viral infection as the model is
produce interleukins such as interleukin-2 (IL-2), IL-4, and
shown in Figure 57–2. A detailed description is presented
in Chapter 58.
bodies specific for that antigen. (Note that the interleukins
are nonspecific; the specificity lies in the T cells and B cells
1. Cell-Mediated Immunity
and is mediated by the antigen receptors on the surface of
In the following example, a bacterium (e.g., M. tuberculosis)
these cells.) The activated B cell proliferates and differenti-
ates to form many plasma cells that secrete large amounts
enters the body and is ingested by a macrophage. The bac-
terium is broken down, and fragments of it called antigens
Although antibody formation usually involves helper T
or epitopes appear on the surface of the macrophage in
association with class II major histocompatibility com-
cells, certain antigens (e.g., bacterial polysaccharides) can
plex (MHC) proteins. The antigen–class II MHC protein of immunoglobulins (antibodies).
activate B cells directly, without the help of T cells, and are
complex interacts with an antigen-specific receptor on the
called T-cell–independent antigens. In this T-cell–
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mebooksfree.com mebooksfree.com mebooksfree.com for the B cell to “class switch” to produce IgG, IgA, and IgE. mebooksfree.com
independent response, only IgM is produced by B cells
surface of a helper T lymphocyte. Activation and clonal
proliferation of this antigen-specific helper T cell occur as
because it requires IL-4 and IL-5 made by the helper T cell
a result of the production of interleukins, the most impor-
tant of which are interleukin-2 (T cell growth factor) and
See Chapter 59 for a discussion of “class switching,” the
gamma interferon (activates macrophages). These acti-
process by which the B cell switches the antibody it pro-
vated helper T cells, aided by activated macrophages,
duces from IgM to one of the other classes.
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