CHAPTER 8: LYMPHATIC AND IMMUNE SYSTEMSDocument Transcript
Chapter 21: Defenses against Disease
I. Organs, Tissues, and Cells of the Immune System
Antibody: A protein produced in response to the presence of an antigen. Each antibody
combines with a specific antigen.
Antigen: A foreign substance, protein or polysaccharide in nature that stimulates the
immune system to react and produce antibodies.
Immunity: The ability to react to antigens so that the body remains free of disease.
Disease: A state of homeostatic imbalance. It can be due to an infection by foreign
agents or failure of the immune system to function properly.
Lymphoid organs contain large numbers of B and T lymphocytes.
A. The primary lymphatic organs are the red bone marrow and thymus gland.
1. Red bone marrow
Site of stem cells that are capable of dividing and producing cells that go
on to differentiate into the various types of blood cells.
Most bones of a child have red bone marrow. In adults, it is only found in
skull, sternum, ribs, clavicle, pelvic bones, and vertebral column.
B and T lymphocytes produced here, B lymphocytes mature here.
2. Thymus gland
Located along the trachea behind the sternum in the upper thoracic cavity.
Produces the hormone thymosin which is involved in the maturation of T
Immature T lymphocytes migrate from the bone marrow through the
bloodstream to the thymus, where they mature.
Only ~5% of these cells ever leave the thymus. If any show the
ability to react with “self” cells, they die. If they have the potential
to attack a foreign cell, they leave the thymus.
Without a thymus, an individual does not reject foreign tissues, blood
lymphocyte levels are drastically reduced, and the body’s response to most
antigens is poor or absent.
B. The secondary lymphatic organs are the spleen, lymph nodes, and other organs, such
as the tonsils.
Located in upper left abdominal cavity just below the diaphragm.
Contains white pulp and red pulp.
White pulp contains B and T lymphocytes.
Red pulp functions to filter the blood that passes through by
removing debris and old, worn-out or damaged rbcs.
2. Lymph Nodes
Small ovoid structures of lymphoid tissue located along lymphatic vessels.
Contain many B and T lymphocytes.
B lymphocytes filter lymph by engulfing pathogens and debris.
T lymphocytes fight infections and attack cancer cells.
Lymph nodes cluster in certain regions of the body (groin, armpits).
Patches of lymphatic tissue located in a ring about the pharynx.
Perform the same functions as lymph nodes.
First to encounter pathogens and antigens that enters the body by way of
the nose and mouth.
II. Nonspecific and Specific Defenses
A. Immunity includes nonspecific defenses and specific defenses.
1. The four nonspecific defenses include barriers to entry, the inflammatory
reaction, natural killer cells, and protective proteins.
a. Barriers to Entry
Skin and mucous membranes lining respiratory, digestive, and
urinary tracts serve as mechanical barriers to entry by pathogens.
Oil gland secretions contain chemicals that weaken or kill certain
bacteria on the skin.
Ciliated cells lining the upper respiratory tract sweep mucus and
trapped particles up into throat to be swallowed or coughed out.
The stomach has an acidic pH which inhibits the growth of or kills
many types of bacteria.
Normal bacteria that reside in the intestine or vagina prevent
pathogens from taking up residence.
b. Inflammatory Reaction
Whenever tissue is damaged by physical or chemical agents or by
pathogens, a series of events occur—inflammatory reaction.
An inflamed area has four outward signs: redness, heat, swelling,
All of these signs are due to capillary changes in the
Aspirin, ibuprofen, and cortisone are anti-inflammatory agents that
counter the inflammatory reaction.
c. Natural Killer (NK) Cells
Large, granular lymphocytes that kill virus-infected cells and
tumor cells by cell-to-cell contact.
Lack specificity and memory.
d. Protective Proteins
The complement system is composed of a number of blood plasma
Complement proteins are activated when pathogens enter
Complement proteins amplify the inflammatory reaction.
Some complement proteins join to form a membrane attack
complex (Big MAC Attack) that produces holes in the
surface of bacteria and some viruses.
Interferons are proteins produced by virus-infected cells as a
warning to non-infected cells in the area.
Binds to receptors of non-infected cells, causing them to
produce substances that interfere with viral replication.
2. Specific Defenses
If non-specific defenses fail to prevent an infection, specific defenses are
Antigens are foreign substances, protein or polysaccharide in nature, that
stimulate the immune system to react.
Pathogens have antigens. Antigens can be components of foreign or
The immune system is able to distinguish self from non-self.
Specific defenses take five to seven days to become fully activated.
Immunity produced as a result of the action of specific defenses usually
lasts some time. We do not ordinarily get the same illness a second time
Specific defenses primarily depend on the action of lymphocytes, which
differentiate into either B lymphocytes (B cells) or T lymphocytes (T
B cells and T cells are capable of recognizing antigens because they have
specific antigen receptors whose shape allows them to combine with
Each B cell and T cell has only one type of receptor-specific.
Receptor-antigen fit is compared to a lock and key.
During our lifetime, we encounter a million different antigens. We need a
great diversity of B cells and T cells to protect us against them.
During maturation, diversification produces a different B cell and/or T cell
for each possible antigen.
B cells give rise to plasma cells, which produce antibodies, which are
capable of combining with and neutralizing a particular antigen.
T cells do not produce antibodies. They differentiate into either helper T
cells, which release chemicals to regulate the immune response, or
cytotoxic T cells, which attack and kill virus-infected cells and tumor
B Cells and Antibody-Mediated Immunity
Each type of B cell carries its specific antibody as a membrane-
bound receptor (B-cell receptor) on its surface.
B cell is activated in a lymph node or spleen when its B-cell
receptor binds to a specific antigen.
The B cell then divides many times.
Most of the resulting cells (clones) become plasma cells, which
circulate in the blood and lymph.
Plasma cells are larger than regular B cells due to their
mass production and secretion of antibodies to a specific
Clonal selection theory: An antigen selects, then binds to the B-
cell receptor of only one type of B cell, and then this B cell clones.
B cell will not divide until its antigen is present. It recognizes
Some cloned B cells do not participate in antibody production, but
remain in blood as memory B cells—long term immunity.
If the same antigen enters the system again, memory B
cells quickly divide and give rise to more plasma cells
capable of quickly producing the correct type antibody.
Once threat of infection has passed, development of new plasma
cells ceases. Those that are present undergo apoptosis.
Apoptosis: Process of programmed cell death. Contributes
to homeostasis by regulating the number of cells present in
the immune system.
B cells are responsible for antibody-mediated immunity because
the various types of activated B cells become plasma cells that
Structure of an Antibody
Antibodies are also called immunoglobulins (Igs)
Typically Y-shaped molecules with two arms.
Each arm has a “heavy” (long) and “light” (short)
Chains have constant regions where the sequence of
amino acids is set.
The constant regions are not identical among all
Chains have variable regions where the sequence of
amino acids varies between antibodies.
Variable region forms antigen-binding site.
Antigen binds with a specific antibody at antigen-
binding site in a lock-and-key manner.
Antigen-antibody complex marks antigen for
Types of Antibodies
There are five classes of circulating antibodies or
These are the major type in blood, lymph, and tissue
Bind to pathogens and their toxins.
Can cross the placenta and are present in breast
Contain 5 Y-shaped structures.
They appear in blood soon after an infection begins
and disappear before it is over.
Good activators of the complement system.
Main type of antibody found in bodily secretions
such as saliva, tears, and breast milk.
Bind to pathogens before they reach the
Role of IgD antibodies is to serve as receptors for
antigens on mature B cells.
IgE antibodies are involved in immediate allergic
responses and are important in the immune system’s
response to parasites.
T Cells and Cell-Mediated Immunity
When a T cell leaves the thymus, it has a unique T-cell receptor
just as B cells have.
T cells though cannot recognize antigen simply present in lymph or
blood without help.
Antigen must be displayed to them by antigen-presenting
When an antigen-presenting cell, usually a macrophage,
engulfs a microbe, it is enclosed within an endocytic
vesicle and broken down to release fragments.
These fragments are antigenic, each of which is then
displayed in the groove of a MHC (major
histocompatibility complex) protein on the cell’s surface.
MHC proteins are called self proteins because they
mark cells as belonging to a particular individual.
If an APC displays an antigen within the groove of a
MHC I protein, the activated T cell will form cytotoxic
If an APC displays an antigen within the groove of a
MHC II protein, the activated T cell will form helper T
Importance of MHC proteins in transplants—specificity
When a donor and recipient are histocompatible, it
is likely a transplant will be successful.
Once a T cell recognizes an antigen, it undergoes clonal
expansion and many copies of the same T cell are
As the infection disappears, the immune reaction wanes,
and the majority of activated T cells become susceptible to
A small number of T cells though remain as memory T
cells. Memory T cells provide protection should the same
antigen enter the body again at a future time.
Types of T cells
Cytotoxic T cells: Destroy antigen-bearing cells.
Have storage vacuoles that contain perforin molecules.
Perforin molecules perforate a plasma membrane,
causing the cell to undergo apoptosis and die.
Helper T cells: Regulate immunity by secreting cytokines, the
chemicals that enhance the response of all types of immune cells.
B cells cannot be activated without T cell help.
HIV infects primarily Helper T cells and inactivates the
Memory T cells remain in the body and can jump-start an
immune reaction when the same antigen re-enters the body.
III. Acquired Immunity
Immunity is acquired naturally through infection or is brought about artificially by medical
A. 2 types of acquired immunity:
1. Active Immunity: Individuals make their own antibodies against an antigen.
2. Passive Immunity: Individuals receive prepared antibodies via an injection.
Sometimes develops naturally after a person is infected with a pathogen.
Active immunity is often induced when a person is well so that future
infection is prevented.
To prevent infections, people can be artificially immunized against them.
Immunization involves the use of vaccines which are substances
that contain an antigen to which the immune system responds.
Vaccines are the pathogens themselves or their products that have
been treated so they are no longer virulent (able to cause disease).
Genetically engineered bacteria can mass-produce proteins from
pathogens. These proteins can be used as a vaccine—Hepatitis B.
After a vaccine is given, immune response is measured by
determining the antibody level in plasma—antibody titer.
After first exposure to a vaccine, a primary response occurs with
no antibodies present and then a slow rise in titer.
A gradual decline in titer follows as antibodies bind to antigen or
just simply breakdown.
After a second exposure to a vaccine, a secondary response occurs
and antibody titer rises rapidly to a level much greater than before.
This is a “booster” because it boosts the antibody titer to a high
Higher antibody titer is now expected to help prevent disease
symptoms even if the individual is exposed to the disease-causing
Active immunity depends on memory B cells and memory T cells
responding to lower doses of antigen.
Active immunity is usually long-lasting, although a booster may be
required every so many years.
Occurs when an individual is given prepared antibodies
(immunoglobulins) to combat a disease.
Since these antibodies are not produced by the individual’s plasma cells,
passive immunity is temporary.
Newborn infants are passively immune to some diseases because mother’s
IgG antibodies have crossed the placenta. These antibodies soon
Breast-feeding prolongs the natural passive immunity an infant receives
from the mother because IgG and IgA antibodies are present in mother’s
Even though passive immunity does not last, it is sometimes used to
prevent illness in a patient who has been unexpectedly exposed to an
A person may be given a gamma globulin injection (serum that
contains antibodies against the agent) taken from an individual
who has recovered from the illness.