Page 515 - Review of Medical Microbiology and Immunology ( PDFDrive )

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PART VII Immunology
504
TABLE 58–2 Cell Surface Proteins That Play an
1
but not both.
Important Role in the Immune Response
CD4 lymphocytes perform the following helper func-
Type of Cells
Surface Proteins
tions: (1) they help B cells develop into antibody-producing
plasma cells; (2) they help CD8 T cells to become activated
CD4, TCR, CD28
Helper T cells
cytotoxic T cells; and (3) they help macrophages kill
Cytotoxic T cells
CD8, TCR
ingested microbes. These functions are performed by three
B cells
IgM, B7
distinct subpopulations of CD4 cells:
2
Class II MHC
Macrophages
(1) The Th-1 cell subpopulation produces gamma
Receptors for class I MHC
Natural killer cells
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interferon that activates macrophages to become more
Class I MHC
All cells other than mature red cells
effective killers of intracellular microbes such as Mycobac-
IgM = immunoglobulin M; MHC = major histocompatibility complex; TCR = T-cell
terium tuberculosis (Figure 58–3). The activated macro-
antigen receptor.
phages have more lysosomal proteases, more hypochlorite,
1
There are many other cell surface proteins that play a role in the immune response,
and more reactive oxygen species such as superoxides, all of
but the proteins listed in this table are the most important for understanding the
which collaborate in the killing of the intracellular microbes.
fundamental aspects of this response.
2
Also, Th-1 cells are an important source of IL-2 which is
Macrophages and other antigen-presenting cells.
3
T-cell growth factor, that is, it stimulates the growth of both
Mature red blood cells do not synthesize class I MHC proteins because they do not
have a functioning nucleus.
CD4 and CD8 T cells. Note also that the cytokines produced by
macrophages. The effector functions are carried out pri-
(see page 512) to produce two subclasses of IgG (namely IgG 1
marily by cytotoxic (CD8-positive) T cells, which kill virus-
infected cells, tumor cells, and allografts. Th-1 cells (e.g., gamma interferon) help B cells to class switch
(2) The Th-2 cell subpopulation performs the B-cell
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helper function primarily by producing IL-4 and IL-5 (see
CD4 & CD8 Types of T Cells
Figure 58–3). IL-4 and IL-5 enhance the growth and dif-
ferentiation of B cells into plasma cells. Also, IL-4 induces
Within the thymus, perhaps within the outer cortical epithe-
lial cells (nurse cells), T-cell progenitors differentiate under
against worms. IL-5 increases the number and activity of
the influence of thymic hormones (thymosins and thymo-
eosinophils which further enhances the host defense
poietins) into T-cell subpopulations. These cells are charac-
against worms. IL-5 also induces class switching to IgA
terized by certain surface glycoproteins (e.g., CD3, CD4, and
thereby enhancing gut immunity.
CD8). All T cells have CD3 proteins on their surface in
(3) The Th-17 cell subpopulation produces IL-17 that
association with antigen receptors (TCR [see later]). The
CD3 complex of five transmembrane proteins is involved
enhance mucosal immunity, especially in the gastrointesti-
with transmitting, from the outside of the cell to the inside,
nal tract (not shown in Figure 58–3).
the information that the antigen receptor is occupied. One
One important regulator of the balance between Th-1
of the CD3 transmembrane proteins, the zeta chain, is linked recruits neutrophils to a site of inflammation. Th-17 cells
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cells and Th-2 cells is IL-12, which is produced by macro-
to a tyrosine kinase called fyn, which is involved with signal
phages. IL-12 increases the number of Th-1 cells, thereby
transduction. The signal is transmitted via several second
enhancing host defenses against organisms that are con-
messengers, which are described in the section on activation
(see later). CD4 is a single transmembrane polypeptide,
Another important regulator is gamma interferon, which
whereas CD8 consists of two transmembrane polypeptides.
inhibits the production of Th-2 cells. CD4 cells make up
They may signal via tyrosine kinase (the lck kinase) also.
about 65% of peripheral T cells and predominate in the
T cells are subdivided into two major categories on the
thymic medulla, tonsils, and blood.
basis of whether they have CD4 or CD8 proteins on their
To mount a protective immune response against a spe-
TABLE 58–3 Main Functions of Helper T cells
(i.e., either Th-1 or Th-2 cells) play a dominant role in the
response. For example, if an individual is infected with M.
tuberculosis and Th-2 cells are the major responders, then
Main Functions Cytokine That Medi- cific microbe requires that the appropriate subpopulation
ates That Function
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humoral immunity will be stimulated rather than cell-
IL-2
Activates the antigen-specific helper T cell
mediated immunity. Humoral immunity is not protective
to produce a clone of these cells
against M. tuberculosis, and the patient will suffer severe
Activates cytotoxic T cells
IL-2
IL-4 and IL-5
Activates B cells
Streptococcus pneumoniae and Th-1 cells are the major
responders, then humoral immunity will not be stimulated
Gamma interferon
Activates macrophages
and the patient will have severe pneumococcal disease.
IL = interleukin.
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