647
CHAPTER SIXTEEN
Lymphatic System and Immunity
f. A helper T cell becomes activated when it
encounters displayed antigens for which it is
specialized to react.
g. Once activated, helper T cells stimulate B cells to
produce antibodies.
h. Cytotoxic T cells recognize foreign antigens on
tumor cells and cells whose surfaces indicate that
they are infected by viruses. Stimulated cytotoxic T
cells secrete perforin to destroy these cells.
i. Memory T cells allow for immediate response
to second and subsequent exposure to the same
antigen.
4. B cells and the humoral immune response
a. B cell activation
(1) Sometimes a B cell is activated when it
encounters an antigen that F
ts its antigen
receptors or more often a B cell is activated
when stimulated by a helper T cell.
(2) An activated B cell proliferates (especially when
stimulated by a T cell), enlarging its clone.
(3) Some activated B cells differentiate further into
memory cells.
(4) Other activated B cells differentiate into
antibody-producing plasma cells.
(5) Antibodies react against the antigen-bearing
agent that stimulated their production
(humoral immune response).
(6) An individual’s diverse B cells defend against
many pathogens.
b. Antibody molecules
(1) Antibodies are soluble proteins called
immunoglobulins.
(2) They constitute the gamma globulin fraction of
plasma.
(3) Each immunoglobulin molecule consists of
four linked chains of amino acids.
(4) Variable regions at the ends of these chains are
specialized into antigen binding sites to react
with different antigens.
c. Types of immunoglobulins
(1) The F
ve major types of immunoglobulins are
IgG, IgA, IgM, IgD, and IgE.
(2) IgG, IgA, and IgM make up most of the
circulating antibodies.
d. Antibody actions
(1) Antibodies directly attach to antigens, activate
complement, or stimulate local tissue changes
that are unfavorable to antigen-bearing agents.
(2) Direct attachment results in agglutination,
precipitation, or neutralization.
(3) Activated complement proteins alter infected
cells so they become more susceptible to
phagocytosis, attract phagocytes, and lyse
foreign cell membranes.
5. Immune responses
a. B cells or T cells F
rst encountering an antigen for
which they are specialized to react constitutes a
primary immune response.
(1) During this response, antibodies are produced
for several weeks.
(2) Some T cells and B cells remain dormant as
memory cells.
6. Phagocytosis
a. The most active phagocytes in blood are
neutrophils and monocytes; monocytes give rise to
macrophages, which remain F
xed in tissues.
b. Phagocytic cells associated with the linings
of blood vessels, in the red bone marrow,
liver, spleen, and lymph nodes constitute the
mononuclear phagocytic system.
c. Phagocytes remove foreign particles from tissues
and body fl
uids.
7. ±ever
a. Viral or bacterial infection stimulates certain
lymphocytes to secrete IL-1, which temporarily
raises body temperature.
b. Physical factors, such as heat or ultraviolet light, or
chemical factors, such as acids or bases, can cause
fever.
c. Elevated body temperature and the resulting
decrease in blood iron level and increased
phagocytic activity hamper infection.
16.9
ADAPTIVE (SPECIFIC) DEFENSES OR
IMMUNITY (PAGE 628)
1. Antigens
a. Before birth, body cells inventory “self” proteins
and other large molecules.
b. After inventory, lymphocytes develop receptors
that allow them to differentiate between nonself
(foreign) and self antigens.
c. Nonself antigens combine with T cell and B cell
surface receptors and stimulate these cells to cause
an immune reaction.
d. Haptens are small molecules that can combine
with larger ones, becoming antigenic.
2. Lymphocyte origins
a. Lymphocytes originate in red bone marrow and are
released into the blood.
b. Some reach the thymus where they mature into T
cells.
c. Others, the B cells, mature in the red bone marrow.
d. Both T cells and B cells reside in lymphatic tissues
and organs.
e. Varieties of T cells and B cells number in the
millions.
f. The members of each variety respond only to a
speciF
c antigen.
g. As a group, the members of each variety form a
clone.
3. T cells and the cellular immune response
a. T cells are activated when an antigen-presenting
cell displays a foreign antigen.
b. When a macrophage acts as an accessory cell, it
phagocytizes an antigen-bearing agent, digests
the agent, and displays the antigens on its cell
membrane in association with certain MHC
proteins.
c. T cells respond to antigens by cell-to-cell contact
(cellular immune response).
d. T cells secrete cytokines, such as interleukins, that
enhance cellular responses to antigens.
e. T cells may also secrete substances that are toxic to
their target cells.
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