Chapter 14: Lymphatic and
Immune Systems
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The Lymphatic System
The lymphatic system has three main
functions:
(1) lymphatic vessels return excess
tissue fluid to the bloodstream,
(2) lacteals in the small intestine receive
and transport lipoproteins, and
(3) lymphoid organs work with the
immune system to combat disease.
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Lymphatic Vessels
The lymphatic vessels are similar in
structure and function to veins;
movement of lymph is dependent upon
skeletal muscle contraction, and
internal valves prevent backflow.
Lacteals collect fat molecules at intestinal
villi and lymphatic capillaries collect
excess tissue fluid at blood capillaries.
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Tissue fluid becomes lymph once it
enters the lymphatic capillaries.
Lymphatic capillaries merge to form
larger lymphatic vessels that carry
lymph to the subclavian veins.
Edema is retention of fluids in the tissues
due to a blockage of the flow of lymph.
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Lymphatic system
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Lymphoid Organs
Lymphoid organs include the lymph
nodes, tonsils, spleen, thymus gland,
and the red bone marrow.
Lymph nodes are located at certain
points along lymphatic vessels and
contain a cortex and a medulla.
The cortex contains nodules where
lymphocytes congregate when fighting
off a pathogen.
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Macrophages are concentrated in the
medulla where they work to cleanse the
lymph.
Tonsils are patches of lymphatic tissue in
the pharynx; they are the first to
encounter pathogens in the nose and
mouth.
Tonsils perform functions similar to
those of the lymph nodes.
The spleen cleanses blood of pathogens
and their toxins.
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The thymus is located behind the sternum
and is the site in which T lymphocytes
mature.
The thymus produces the hormone
thymosin that appears to cause T
lymphocytes to mature; thymosin may
play other roles in immunity.
The red bone marrow is the site where all
types of blood cells are produced by
stem cells.
B lymphocytes mature in the red bone
marrow.
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The lymphoid organs
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Nonspecific Defenses
The immune system includes cells and
tissues that are responsible for
immunity.
Immunity is the body’s ability to defend
against infection and involves
nonspecific and specific defenses.
The nonspecific defenses are effective
against many types of infectious
agents.
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Barriers to Entry
Skin and mucous membranes lining the
respiratory, digestive and urinary tracts,
oil glands in the skin, ciliated cells that
sweep particles in mucus, and an acidic
stomach all contribute to keeping
pathogens from entering the body.
Beneficial bacteria in the intestines and
vagina also prevent pathogens from
taking up residence.
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Inflammatory Reaction
The inflammatory reaction involves
redness, heat, swelling, and pain.
The release of histamine and kinins from
damaged tissue cells and from mast
cells causes redness and swelling.
The swollen area and kinins stimulate
free nerve endings, causing the
sensation of pain.
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Macrophages migrate to the site of injury
and can engulf pathogens and also
release colony-stimulating factors that
cause the bone marrow to release more
white blood cells.
Anti-inflammatory drugs combat chronic
inflammation by acting against
chemical mediators released by white
blood cells.
The presence of pus indicates the body
is trying to overcome the infection.
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Inflammatory reaction
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Natural Killer Cells
Natural killer cells kill virus-infected cells
and tumor cells by cell-to-cell contact.
They are large, granular lymphocytes
with no specificity and no memory.
Their number is not increased by prior
exposure to that kind of cell.
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Protective Proteins
The complement system, or complement,
refers to a number of plasma proteins
that assist nonspecific immunity.
A small amount of activated complement
protein is needed to activate a cascade
of other proteins.
Complement is activated when
pathogens enter the body.
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Complement amplifies the inflammatory
reaction by attracting phagocytes and
promoting phagocytosis.
Some complement proteins join to form
pores in the surface of bacteria and
cause them to burst.
Interferon is a protein produced by virusinfected cells that prepares noninfected cells for attack and interferes
with viral replication; it is specific to
species.
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Action of the complement
system against a bacterium
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Specific Defenses
An antigen is any foreign substance that
stimulates the immune system to react
to it.
The body does not consider its own
proteins foreign; therefore the immune
system must distinguish self from
nonself.
Lymphocytes have a large number of
antigen receptors.
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Specific defenses require B lymphocytes
(B cells) and T lymphocytes (T cells),
which are both produced in the bone
marrow; however, T cells mature in the
thymus, while B cells mature in bone
marrow.
B cells give rise to antibodies that are
shaped like antigen receptors and are
capable of combining with and
neutralizing antigens.
T cells do not produce antibodies but
instead attack foreign antigens directly.
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B Cells and Antibody-Mediated
Immunity
A toxin is a chemical produced by certain
bacteria that is poisonous.
As a B cell encounters a bacterial cell or a
toxin with a specific antigen in a lymph
node or spleen, it is activated to divide.
The resulting cells are plasma cells,
mature B cells that mass-produce
antibodies.
Defense by B cells is thus called antibodymediated immunity.
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According to the clonal selection theory, it
is the antigen that selects which
lymphocyte will undergo clonal
expansion, mass producing lymphocytes
bearing matching antigen receptors.
Some members of the clone become
memory B cells that remain in the body
to divide and produce more lymphocytes
able to secrete antibodies if the same
antigen is encountered at a later date.
When infection passes, plasma cells
undergo apoptosis.
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Clonal selection theory and B
cells
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Structure of IgG
The most common type of antibody, the
IgG antibody, is a Y-shaped molecule
that has two binding sites for a specific
antigen.
Antigen-antibody complexes often mark
the antigen for destruction by
neutrophils or macrophages, or they
may activate complement.
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Structure of IgG
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Other Types of Antibodies
There are five types of antibodies:
IgG – the main type in circulation, binds
to pathogens, activates complement,
and enhances phagocytosis
IgM – the largest type in circulation,
activates complement and clumps cells
IgA – found in saliva and milk, prevents
pathogens from attaching to epithelial
cells in digestive and respiratory tracts
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IgD – on surface of immature B cells, its
presence signifies the readiness of a B
cell
IgE – found as antigen receptor on
basophils in blood and on mast cells in
tissues, responsible for immediate
allergic response and protection
against certain parasitic worms.
The different classes of antibodies vary
in structure.
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T Cells and Cell-Mediated
Immunity
T cells mature in the thymus.
Like B cells, each T cell bears a specific
receptor, however, for a T cell to
recognize an antigen, the antigen must
be presented by an antigen-presenting
cell (APC).
When a viral or cancer cell antigen is
presented, the antigen is first linked to
a major histocompatibility complex
(MHC) protein.
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Human MHC proteins are called HLA
(human lymphocyte-associated)
antigens; HLA antigens are self proteins
that identify tissues as belonging to the
body (the basis of tissue transplant
rejection).
Once a T cell’s antigen receptor matches
up to its specific antigen presented to it
by a macrophage (the APC), the T cell
becomes activated and secretes
cytokines and undergoes clonal
expansion.
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Clonal selection theory and T
cells
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Types of T Cells
Cytotoxic T cells kill infected cells that
bear a foreign antigen on contact using
perforin molecules; cytotoxic T cells
provide cell-mediated immunity.
Helper T cells stimulate other immune
cells and produce cytokines.
Some T cells are memory T cells that will
jump-start an immune reaction upon reinfection.
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Cell-mediated immunity
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Induced Immunity
Immunity occurs naturally by infection or
is induced by medical intervention.
The two types of induced immunity are
active immunity and passive immunity.
In active immunity, the individual
produces the antibodies against an
antigen.
In passive immunity, the individual is
given prepared antibodies.
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Active Immunity
A person naturally develops active
immunity after infection.
Immunization involves the use of
vaccines, substances that contain an
antigen to which the immune system
responds.
Vaccines are available to induce longlived active immunity in a well person.
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After exposure to a vaccine, which is a
non-virulent disease agent, antibodies
are produced.
With a booster shot or second exposure,
the antibody titer rises to a much
higher level.
Active immunity is long-lived because
there are memory B cells and memory
T cells that will respond to lower doses
of antigen in the body.
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Active immunity due to
immunizations
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Passive Immunity
Passive immunity occurs when an
individual is given prepared antibodies.
For example, a newborn has antibodies
that passed from its mother through
the placenta.
Breast-feeding passes antibodies from
mother to child.
However, passive immunity is short-lived
since the antibodies were not produced
by the person’s own B cells.
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Passive immunity
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Cytokines and Immunity
Cytokines are signaling molecules
produced by T lymphocytes,
monocytes, and other cells.
Both interferon and interleukins are
cytokines used to improve a person’s
own T cell performance in fighting
cancer.
Interleukins show promise in the
treatment of chronic infectious
diseases.
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Monoclonal Antibodies
All plasma cells derived from the same B
cell secrete an identical antibody.
B lymphocytes can therefore be exposed
to a particular antigen and will produce
monoclonal antibodies to the specific
antigen.
Activated B lymphocytes are fused with
myeloma cells and these hybridomas
produce only one type of antibody.
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Production of monoclonal
antibodies
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Immunity Side Effects
The immune system usually protects us
from disease because it can distinguish
self from nonself.
Sometimes, however, it responds in a
manner that harms the body.
Examples include allergies, tissuerejection reactions, or autoimmune
diseases.
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Allergies
Allergies are hypersensitivities to
substances such as pollen or animal
dander that are normally not harmful.
Weak antigens such as these are called
allergens.
The response itself can cause some
degree of tissue damage.
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Immediate Allergic Response
An immediate allergic response can occur
within seconds after contacting an
antigen.
Anaphylactic shock is a severe reaction
characterized by a sudden life-threatening
drop in blood pressure.
Immediate allergic responses are caused by
IgE antibodies attaching to mast cells and
basophils; these cells then release
histamine which causes the symptoms of
allergies, some of which are severe.
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Delayed Allergic Response
Delayed allergic responses are started by
memory T cells at the site of the
allergen.
The response is regulated by cytokines
secreted by T cells and macrophages.
Examples of delayed allergic responses
include the skin test for tuberculosis
and contact dermatitis from poison ivy,
jewelry, and other possible irritants.
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Blood-Type Reactions
Illness and death sometimes resulted from
the first attempts at blood transfusions.
It was later discovered that only certain
types of blood are compatible because
red blood cell membranes carry proteins
or sugar residues that are antigenic to
recipients.
The ABO blood system represents a series
of antigens on red blood cells that
denote blood type.
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ABO System
The ABO blood typing system is based on
the presence of two antigens on the
surface of red blood cells; antigen A and
antigen B.
Blood types include A, B, or AB, or type O,
which has no antigens.
In the plasma there are two possible
naturally-occurring antibodies: anti-A
and anti-B.
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If the corresponding antigen and
antibody are put together, clumping, or
agglutination, occurs; in this way the
blood type of the individual may be
determined.
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Blood typing
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Rh System
The Rh factor is also a blood type; a
person with this antigen on their red
blood cells is Rh positive (Rh+); those
without it are Rh negative (Rh-).
Rh factor is important during pregnancy
because an Rh- mother may form
antibodies to the Rh antigen during the
pregnancy or at the birth of a child who
is Rh+.
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These antibodies can cross the placenta
to destroy the red blood cells of any
subsequent Rh+ child, causing
hemolytic disease of the newborn.
A Rho-Gam injection uses anti-Rh
antibodies to attack Rh+ cells before
they can stimulate the mother to
produce her own antibodies.
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Hemolytic disease of the
newborn
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Tissue Rejection
Tissue rejection occurs when cytotoxic T
cells bring about the destruction of
foreign tissue in the body.
A close match between donor and
recipient can reduce rejection.
Immunosuppressive drugs act by
inhibiting the response of T cells to
cytokines, but can result in kidney
damage.
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Autoimmune Diseases
Autoimmune disease occurs when
cytotoxic T cells or antibodies
mistakenly attack the body’s own cells
as if they bear foreign antigens.
Autoimmune diseases include:
myasthenia gravis, multiple sclerosis,
systemic lupus erythematosus, and
rheumatoid arthritis.
It has been suggested that type I diabetes
and heart damage after rheumatic fever
are autoimmune diseases.
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Chapter Summary
Lymphatic vessels form a one-way
system that transports lymph from
tissues and fat from lacteals to specific
cardiovascular veins.
Lymphoid organs include the red bone
marrow, spleen, thymus, and lymph
nodes that all play critical roles in
defense mechanisms.
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Immunity consists of nonspecific and
specific defenses to protect against
disease.
Nonspecific defenses consist of barriers
to entry, the inflammatory reaction,
natural killer cells, and protective
proteins.
Specific defenses involve two types of
lymphocytes: B lymphocytes and T
lymphocytes.
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Medically induced immunity involves use
of vaccines to achieve long-lasting
immunity and use of antibodies to
provide temporary immunity.
While immunity protects us, it is also
responsible for undesirable effects,
such as allergies, autoimmune
diseases, and tissue rejection.
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