Overview of the Lymphatic System:
cells, tissues, and organs responsible for defending the
body against both environmental hazards (pathogens)
and internal threats (cancer)
 Pathogens: viruses, bacteria, fungi, and
parasites capable of not only surviving
but thriving inside our bodies
 Immune response: cells of the immune
system that provide a specific response
as a defense against pathogen
 Immunity: Ability to resist infection
and disease through the activation of
specific defenses.
Organization of the Lymphatic
System
 Lymph: plasma-like fluid
 Lymphatic vessels: begin in
tissues and end in connections
to veins
 Lymphoid tissues and organs
 Lymphocytes and other cells
The body has between 501 and 700 lymph
nodes (the number of nodes varies from
individual to individual).
About half of the nodes are in the middle of
your body (stomach or abdominal cavity).
The lymph nodes near your armpits and groin
have about 100 nodes.
Functions of the Lymphatic System
 Produce, maintain and distribute lymphocytes
 Produced inside the lymphoid tissues and organs
 Tonsils, spleen, thymus, red bone marrow, appendix
 Provide defense against infections and environmental
hazards
 Must be able to detect problems
 Must be able to reach the infection or injury site

Circulate in the blood
Lymphatic Vessels
 Lymphatic vessels carry lymph from the tissues to the
venous (veins) system.
 Vessels have valves to allow entrance into the vessels for
fluids, proteins, viruses and bacteria but prevents them
from flowing back into the intracellular spaces
 Lymphatic capillaries are the smallest of these vessels
 Present in almost every tissue and organ in the body
Lymphatic Vessels
 Occur in association with blood vessels but are
smaller and more numerous
 Major lymph collecting vessels
 Superficial lymphatics: located in the deep
subcutaneous layer of the skin and in the lining
of the digestive, respiratory, urinary, and
reproductive tracts
 Deep lymphatics: larger, accompany deep
arteries and veins in the limbs and trunk
 Deep and superficial come together to form
larger vessels called trunks
 These empty into two large collecting vessels
Lymphatic Vessels
 Thoracic duct
begins below the diaphragm
 collects lymph from the left
side of the head, neck, thorax
and inferior portion of the
body
 empties it into the left
subclavian vein to reenter
into venous circulation
 Right lymphatic duct
 formed from the merging of
major blood vessel trunks on
the right side of the body
 empties into the right
subclavian vein.
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Lymphodema
 Blockage of lymphatic
drainage from the
limbs
 Fluid accumulates in
the limb and gradually
becomes swollen
 Can be permanent if
the condition persists
 Dangerous if an
infection develops in
that area
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Why?
Lymphocytes
“Lymph cells”
 3 classes
 Each has it’s own
biochemical and functional
characteristics
 T cells (thymus-dependant)
 B cells (bone marrow-
derived)
 NK cells (natural killer)
 “pit bulls of the lymphatic
system
T cells: 80% of lymphocytes
 Cytotoxic T cells
 Attack foreign cells or body cells
infected with a virus
 Helper T cells
 Suppressor T cells
 Inflammatory T cells
 Stimulate the activation and
function of both T cells and B cells
 Inhibit the activation and function
of both T cells and B cells
 Stimulate regional inflammation
and local defenses in injured tissue
Other lymphocytes
B cells
NK cells
 10-15 % of lymphocytes
 Can differentiate into plasma
 5-10 % of lymphocytes
cells
 Responsible for producing
antibodies
 These bind to specific
chemical targets called
antigens
 Antigens are typically
pathogens, parts of pathogens
 Some lipids,
polysaccharides and nucleic
acids can stimulate antigen
body production
lymphocytes
 Attack foreign cells, normal
cells infected with viruses, and
cancer cells (often Dr.s
measure #s of these to
measure if cancer cells are
growing or declining)
 Continuous “policing” is called
immunological surveillance
 Also known as large granular
Life span and circulation of lymphocytes
 They are not evenly distributed throughout the blood
 T cells move around the body quickly
 B cells are much slower
 Majority of lymphocytes have long life spans (4 years +)
 Some may live up to 20 years
 You maintain your lymphocyte
populations by making new
lymphocytes in bone marrow
and lymphiod tissue
Lymphocyte Production
 Lymphopoiesis (lymphocyte production) involves
bone marrow, thymus, and peripheral lymphoid
tissues.
 Lymphocytes originate from stem cells
 Division of stem cells in the bone marrow that
produce immature B and NK cells, as they mature they
begin to circulate in the blood stream.
 Second division of stem cells migrates to the
thymus
 In the thymus these cells produce various kinds of T
cells, when they mature they enter the blood
stream.
Lymphoid Tissues:
Connective tissue dominated by
lymphocytes
• Lymphoid nodule:
lymphocytes are densely
packed
• Large clusters of nodules exist
Tonsils: most people have 5
• When the pair in the base of
the pharynx become infected
we get tonsillitis
• Germinal center: central zone
of the nodule where
lymphocytes divide
•
Lymphoid Tissues: dominated by lymphocytes
 MALT: Mucosa-Associated Lymphoid Tissue
 Collection of lymphoid tissue linked with the digestive tract
 Peyer’s patches are cluster of nodules in the lining of intestines
Lymphoid Organs
 Lymph nodes: small
lymphoid organ surrounded
by connective tissue
 Bean shaped
 Hilus is the indentation of
the “bean” where nerve and
blood flow reach the node
 Lymph flow: through open
passageways with incomplete
walls called sinus
 Then through the outer
cortex of the node
 Then continues to the deep
cortex and into the core or
medulla of the lymph node
Lymph Node Function
 Purifies lymph before it reaches
the vessels
 Like a water filter
 Early warning system for the body

Pathogens stimulate
macrophage/lymphocyte production in
nodes
 Lymph glands at the groin, axilla
(armpit), and base of the neck act
as guards to protect the trunk for
pathogens

Minor injury/infection causes “swollen
glands” when lymph nodes respond to
injury/infection by increasing production
Thymus
 Pink, grainy organ located
behind the sternum
 Produce T cells
 Surrounding blood vessels
allow T cells to enter the
blood stream
 Review:
 Thymus also produces
hormones (endocrine
system)

Thymosin is a hormone
that promotes
development of
lymphocytes
Spleen
 Largest collection lymphoid tissue
 Provides same services for the blood that lymph nodes
do for the lymph
 Remove abnormal blood cells/components by
phagocytosis (think of pac-man)
 Storage of iron recycled from red blood cells
 Initiation of immune response by B cells and T cells in
response to antigens in the blood
Spleen
 Blood circulates through and
allows phagocytes in the spleen to
engulf damaged or infected cells in
the blood
 Infection (such as mono) can cause
the spleen to become enlarged
 The spleen is fragile
 When it is enlarged any blow to
the left side of the body can cause
it to rupture
Non-specific defenses: prevent the approach,
deny entry, or limit the spread of microorganisms or other
environmental hazards.
 Categories
 1. Physical barriers
 2. Phagocytes
 3. Immunological surveillance
 4. Interferons
 5. Complement
 6. Inflammatory response
 7. Fever
Physical Barriers
 Keep hazards outside the body (example: hair on
your head helps keep mosquitoes off)
 In order for a pathogen to cause trouble it has to
enter the body.
 Cross the epithelium (skin) through a mucus membrane
(mouth, nose, eyes, etc)
 Accessory structures help with the barrier (eyelashes, hair,
glands that secrete oil or chemicals/enzymes)
Phagocytes: engulf pathogens and debris
 Microphages (small)
 Circulate in the blood, leave to travel into
tissues to engulf and destroy
pathogens/antigens
 Macrophages (big)
 Several types, in almost every tissue of the
body
 Work by
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Engulfing particle and destroying it with and
enzyme
Bing or remove particle from fluid and receive
help from other cells to destroy it
Releasing toxic chemicals to destroy particle
 Fixed macrophages: permanent residents
of a tissue
 Free macrophages: mobile and travel
through the body to arrive at injury sites
Phagocytes:
 Movement and phagocytosis
 Free and fixed macrophages
characteristics
 Both can move through
capillary walls
 Both may be attracted or
repelled by chemicals ins the
surrounding fluid
(chemotaxis)
 Both use adhesion
(attachment of the phagocyte
to its target) and then
formation of a vesicle
(capsule) that is digested with
help of an enzyme
Immunological surveillance: destruction of
abnormal cells by NK cells
 The immune system usually
ignores your body’s own cells
unless they have become
abnormal in some way (such
as mutated cells or cells
infected with a virus)
 mutations-can lead to what?
 Immunological
surveillance:
 NK cells monitor tissues
looking for and destroying
abnormal cells in any tissue of
the body
 NK cells are highly versatile
 Respond very rapidly
Immunological surveillance:
 Cancer cells’ membranes
contain proteins called
tumor-specific antigens
 NK cells recognize them as
abnormal and destroy them
 However, some cancer cells
go undetected through
immunological escape:
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Escape destruction by: avoiding
detection, covering their
antigens somehow, or killing NK
cells
Once escaping detection, cells
can multiply and spread
Interferons: chemical messengers that
coordinate defenses against viruses
 Interferons are small proteins
released by lymphocytes,
macrophages, and tissues
infected with a virus
 Trigger the production of
antiviral proteins which
interfere with viral replication
inside the cells
 Stimulate the activities of
macrophages and NK cells
Complement: chemical messengers that
coordinate defenses against viruses
 11 special complement proteins: complement the actions of
antibodies
 Complement Activation: Classical Pathway (rapid and
effective)
 One complement protein binds to an antibody molecule
already attached to its antigen
 It then acts as an enzyme starting a series of reactions with
other complement proteins
 Complement: Alternative Pathway (slower and less effective)
 Occurs when there is no antibody molecule
 Several complement proteins interact in the plasma to defend
against bacteria, parasites, and virus infected cells
Complement: chemical messengers that
coordinate defenses against viruses
 Effects of Complement Activation
 Stimulation of Inflammation

Enhance the release of histamine to accelerate inflammation
 Attraction of Phagocytes: to increase destruction of pathogens
 Enhancement of Phagocytosis: complement proteins make
target cells easier to engulf
 Destruction of Target Cell Membranes: so they are easier to
destroy
Inflammatory response: local response to
injury of infection at the tissue level (limits spread of injury, combats infection)
 5 Signs of inflammation:
 Swelling
 Redness
 Heat
 Pain
 Loss of function
 Effects
 Injury is temporarily repaired and
pathogens are kept out
 Spread of pathogens away for the
injury is slowed
 Defenses are mobilized to
overcome pathogens and allow for
permanent repairs
Inflammatory response:
 Tissue response to injury (in a nut shell)
 Mast cells release histamine in to the blood stream to start the
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inflammatory response
Increases cell permeability and increases blood flow by
vasodilation (widening of the vessels)
Clot forms around the injured area
Complement activation breaks down bacterial cell walls and
attracts phagocytes
Special defenses are activated
Macrophages engulf debris (dead cells, dirt etc) and pathogens
Fibroblast are stimulated to repair damaged tissue
Over time the clot is broken down and the tissue is repaired to
normal
Tissue death (necrosis) or infection occurs if the
immunological response fails
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Pus is accumulation fluid with dead cells and necrotic tissue
Abscess is accumulation of pus in an enclosed area
Inflammation (itis)
Fever: elevation of body temperature that
accelerates tissue metabolism and defenses
 Maintenance of a body temperature of greater than 99
degrees F
 A response to a variety of stimuli including pathogens
 Within limits it can be beneficial
 Inhibits some viruses and bacteria
 Increases cell metabolism (for each degree increase metabolic rate
increases by 10%)
 Cells move faster, reactions are quicker
 Leads to faster defenses and repair
An Antigen
 An antigen is any substance that
causes your immune system to
produce antibodies against it.
 Substance may be from the
environment or formed within the
body.
 The immune system will kill or
neutralize any antigen that is
recognized as a foreign and
potentially harmful invader.
 The term originally came from
antibody generator and was a
molecule that binds specifically
to an antibody
Specific Defenses
 Cell mediated immunity: defends against
abnormal cells and pathogens in the body
 Responsibility of T cells
 T cells cannot respond to antigens in solution
 Antibody mediated immunity: defends against antigens
and pathogens in body fluids
 Responsibility of B cells
 Cannot cross cell membranes
Forms of Immunity
 Innate: genetically determined
 Present at birth
 No relationship to exposure to antigen
 Acquired: not present at birth
 after being exposed to a specific antigen
 Active: develops after exposure to an antigen as a
consequence of the immune response
 Naturally acquired active immunity: begins after birth,
constantly changes as you are exposed to new antigens
 Induced active immunity: stimulation of antibodies
under controlled situations: vaccination
Forms of Immunity
 Passive immunity: produced by the transfer of
antibodies from another source
 Naturally acquired: when a child is protected by a
mother’s antibodies in utero and in breast milk
 Induced: antibodies are administer to fight infection
or prevent disease (like when you receive antibodies
after exposure to bacteria)
Properties of Immunity
 Specificity: defense activated by a specific antigentargets that antigen and nothing else.
 Shape/size of the antigen determines what lymphocyte
responds.
 Versatility: your immune system encounters tens of
thousands of antigens in you lifetime, there is no way to
know which it will encounter.
 The diversity of lymphocytes allows the body to be versatile in
its defenses.
 When a specific lymphocyte is activated by and antigen it
begins to divide and make clones.
Properties of Immunity
 Memory: exists because when lymphocytes
divide one group is allowed to destroy the
antigen, and one group remains inactivated so it
can be used if you come into contact with the
same antigen again.
 Tolerance: happens when an antigen does not
stimulate an immune response, such as in a
constant exposure to an antigen
The Creepy Virus
 A virus particle attaches to a host cell.
 The particle releases its genetic
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instructions into the host cell.
The injected genetic material recruits the
host cell's enzymes.
The enzymes make parts for more new
virus particles.
The new particles assemble the parts into
new viruses.
The new particles break free from the
host cell. (which usually dies)
Each virus inside it goes on to find new
hosts--repeat
Primary Responses to Antigen Exposure
 Primary response: initial response to an antigen
 Because the presence of an antigen activates B cells
that then have to differentiate into plasma, the
primary response takes time to develop
 As plasma cells differentiate the concentration slowly
rises and does not peak until about 2 weeks after
exposure to the antigen
 Often this is slow response allows the initial exposure
to the antigen to cause an infection
Secondary Responses to Antigen Exposure
 Secondary response: extensive and prolonged immune
response when an antigen is introduced more than once.
 Response is heightened because there is an increased number
of memory cells ready to destroy the antigen
 B cells do not have to take time to differentiate into plasma
because that has already been done with the first exposure, so
response is much quicker
 This response often prevents exposure of the antigen a second
time from causing an infection
Immune Disorders: because of the complexity of the
immune response, there is opportunity for things to go wrong
Autoimmune disorders
 The immune system normally ignores (self-antigens)
antigens found in the body normally
 Autoantibodies are B cells that activate against self
antigens
 Rheumatoid arthritis occurs when autoantibodies
attack connective tissue in the joints
 Diabetes can be caused by autoantibodies attaching
cells in the pancreas that control insulin
Immune Disorders Cont:
Immunodeficiency disease
 Result from:
 Problems in the development of the lymphoid organs/tissues
 Viral infections such as HIV that suppresses immune function
 Treatment of exposure to immunosuppressive agents
 Severe combined immunodeficiency disease (SCID): persons
born SCID fail to produce cell or antibody mediated
immunity.
 even mild infections can be fatal
 Often have to be isolated (“bubble boy”)
 Immunosuppressive drugs: used to prevent patients from
rejecting transplants
 Can destroy stem cells and lymphocytes and lead to complete
immunological failure
Allergies
 Inappropriate or excessive immune responses to
antigens
 Can damage other cells in the process of destroying antigens
 Can trigger massive inflammation-Mast cells release histamine that
starts the inflammatory process
 Allergens: antigens that trigger allergic reactions
 Antihistamine: drug that blocks the action of
histamine
 Rapid and especially severe response to
an antigen
 First response is typically mild or
unnoticeable
 Second response: histamine and heparin
from mast cells are released to produce
massive inflammation
 T cells and macrophages are drawn to the
area and further the response
 It may be isolated in a tissue, if it enters the
blood stream it can be lethal
Anaphylactic
Shock
Immediate
Hypersensitivity
 Anaphylaxis: circulating antigens
affect mast cell throughout the body
 Massive swelling
 Contraction of muscles along the
respiratory tract
 Vasodilatation can lead to collapse or
anaphylactic shock
 Antihistamines can prevent some of
these symptoms if administered quickly
Integration with other systems
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Gets help from:
Helps out:
CNS innervates the lymph system  Every system by surveying
and spleen
all tissues and destroying
any potentially harmful
Circulatory system helps distribute
invaders
lymphocytes
 Helps with fluid
Skeletal system produces
circulation throughout the
lymphocytes in red marrow
body
Endocrine system hormones help
stimulate lymphocyte production
Integumentary system is a physical
barrier and home to mast cells