528 Chapter 15
SUMMARY
2. Killer T lymphocytes are responsible for transplant
rejection and for the immunological defense against
fungal and viral infections, as well as for defense
against some bacterial infections.
3. Helper T lymphocytes secrete cytokines that stimulate
activity of killer T lymphocytes and B lymphocytes.
4. Regulatory T lymphocytes (previously called suppressor
T lymphocytes) inhibit the immune response, helping to
prevent overactive immune responses that could cause
disease.
5. The T lymphocytes secrete a family of compounds
called lymphokines that promote the action of
lymphocytes and macrophages.
6. Receptor proteins on the cell membrane of T
lymphocytes must bind to a foreign antigen in
combination with a histocompatibility antigen in order
for the T cell to become activated.
7. Histocompatibility antigens, or MHC molecules,
are a family of molecules on the membranes of cells
that are present in different combinations in different
individuals.
C. Antigen-presenting cells, such as macrophages and dendritic
cells, partially digest a foreign protein, such as a virus, and
present the antigens to the lymphocytes on the surface in
combination with class-2 MHC antigens.
1. Helper T lymphocytes require such interaction with
antigen-presenting cells in order to be activated by a
foreign antigen; when activated in this way, the helper T
cells secrete interleukin-2.
2. Interleukin-2 stimulates proliferation of killer T
lymphocytes that are specific for the foreign
antigen.
3. In order for the killer T lymphocytes to attack a victim
cell, the victim cell must present the foreign antigen in
combination with a class-1 MHC molecule.
4. Interleukin-2 also stimulates proliferation of B
lymphocytes, and thus promotes the secretion of
antibodies in response to the foreign antigen.15.4 Active and Passive Immunity 514
A. A primary response is produced when a person is first
exposed to a pathogen. A subsequent exposure results in a
secondary response.
1. During the secondary response, IgM antibodies
are produced slowly and the person is likely to
get sick.
2. During the secondary response, IgG antibodies are
produced quickly and the person is able to resist the
pathogen.
3. In active immunizations, the person is exposed to
pathogens of attenuated virulence that have the same
antigenecity as the virulent pathogen.
4. The secondary response is believed to be due to the
development of lymphocyte clones as a result of
the antigen-stimulated proliferation of appropriate
lymphocytes.15.1 Defense Mechanisms 494
A. Nonspecific (innate) defense mechanisms include barriers to
penetration of the body, as well as internal defenses.
- Phagocytic cells engulf invading pathogens.
- Interferons are polypeptides secreted by cells infected
with viruses that help to protect other cells from viral
infections. - The nonspecific (innate) immune system responds to
pathogen associated molecular patterns (PAMPs) by
interaction of foreign molecules with several receptors,
called toll-like receptors, which recognize these
uniquely foreign molecules.
B. Specific (adaptive) immune responses are directed against
antigens. - Antigens are molecules or parts of molecules that are
usually large, complex, and foreign. - A given molecule can have a number of antigenic
determinant sites that stimulate the production of
different antibodies.
C. Specific immunity is a function of lymphocytes. - B lymphocytes secrete antibodies and provide humoral
immunity. - T lymphocytes provide cell-mediated immunity.
- The thymus and bone marrow are the primary lymphoid
organs, which produce lymphocytes that seed the
secondary lymphoid organs.
D. Specific and nonspecific immune mechanisms cooperate in
the development of local inflammation.
15.2 Functions of B Lymphocytes 503
A. There are five subclasses of antibodies, or immunoglobulins:
IgG, IgA, IgM, IgD, and IgE.
- These subclasses differ with respect to the polypeptides
in the constant region of the heavy chains. - Each type of antibody has two variable regions that
combine with specific antigens. - The combination of antibodies with antigens promotes
phagocytosis.
B. Antigen-antibody complexes activate a system of proteins
called the complement system. - This results in complement fixation, in which
complement proteins attach to a cell membrane and
promote the destruction of the cell. - Free complement proteins promote opsonization and
chemotaxis and stimulate the release of histamine from
tissue mast cells.
15.3 Functions of T Lymphocytes 507
A. The thymus processes T lymphocytes and secretes hormones
that are believed to be required for an effective immune
response of T lymphocytes throughout the body.
B. There are three subcategories of T lymphocytes.
- Killer T lymphocytes kill victim cells by a mechanism
that does not involve antibodies but that does require
close contact between the killer T cell and the victim cell.