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BIOH122
Human Biological Science 2
Session 7
Lymphatic System &
Immune System
Bioscience Department
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Session Plan
o Functions and Structure of the lymphatic system
o Lymphatic vessels and Lymph Flow
o Lymphatic organs and tissues
• Primary lymphatic organs
• Secondary lymphatic organs and tissues
o Immune System Overview
• Innate Immunity :1st Line of Defence
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Functions and Structure of the
lymphatic system
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Lymphatic System
o The lymphatic system contributes to homeostasis by
draining interstitial fluid as well as providing the
mechanisms for defence against disease.
o Functions:
• Drains excess interstitial fluid: Lymphatic vessels drain excess
interstitial fluid from tissue spaces and return it to the blood.
• Transports dietary lipids: Lymphatic vessels transport lipids and
lipid-soluble vitamins (A, D, E, and K) absorbed by the
gastrointestinal tract.
• Carries out immune responses: Lymphatic tissue initiates highly
specific responses directed against particular microbes or
abnormal cells.
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Lymphatic System
o Structure: The lymphatic system consists of
• Lymph: the fluid
• Lymphatic vessels: transport the lymph
• Lymphatic organs and tissue: made of specialized form of
reticular connective tissue that contains large numbers of
lymphocytes
• Red bone marrow: the site of lymphocyte production
o Interstitial fluid and lymph are very similar.
o Their major difference is location.
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Lymphatic System
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Lymphatic vessels and Lymph Flow
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Lymphatic Vessels
o Lymph capillaries: Lymphatic vessels begin as blindended lymph capillaries in tissue spaces between cells.
o Lymphatic vessels: Lymph capillaries merge to form
larger vessels, called lymphatic vessels, which convey
lymph into and out of structures called lymph nodes
o Lymph trunks: Lymphatic vessels exit lymph nodes in a
particular region of the body, they unite to form lymph
trunks
o Lymph Ducts: Lymph passes from lymph trunks into two
main channels, the thoracic duct and the right lymphatic
duct, and then drains into venous blood.
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Lymphatic Capillaries
o Structure:
• slightly larger diameter than blood capillaries
• overlapping endothelial cells which work as one-way
valves for fluid to enter the lymphatic capillary when
tissue pressure is high.
o Anchoring filaments: Attached to the lymphatic
capillaries, contain elastic fibers
• Attach endothelial cells to surrounding tissue and
• Keep tube from collapsing under outside pressure
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Lymphatic Capillaries
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Lymphatic Capillaries
o Location: throughout the body along the systemic
circulation
o Exception: not present in avascular tissue (cartilage,
epidermis and cornea), the central nervous system,
portions of the spleen, and red bone marrow.
o Lacteals: specialized lymphatic capillaries in the
villus of the small intestine, transport digested
fats from the small intestine into the blood.
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Lymph Trunks
o Lymph trunks: lymphatic vessels exit lymph nodes in a
particular region of the body, they unite to form lymph
trunks.
• Lumbar trunks: drain lymph from the lower limbs, the wall and
viscera of the pelvis, the kidneys, the adrenal glands, and the
abdominal wall.
• Intestinal trunks: drains lymph from the stomach, intestines,
pancreas, spleen, and part of the liver.
• Broncho mediastinal trunks: drain lymph from the thoracic wall,
lung, and heart
• Subclavian trunks: drain the upper limbs
• Jugular trunks: drain the head and neck.
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Lymph Ducts
o Lymph Ducts: Lymph passes from lymph trunks into two
main channels, the thoracic duct and the right lymphatic
duct, and then drains into venous blood.
o The thoracic duct:
• is the main collecting duct of the lymphatic system
• begins as a dilation called the cisterna chyli.
• receives lymph from the left side of the head, neck, and chest,
the left upper extremity, and the entire body below the ribs.
• drains lymph into venous blood via the left subclavian vein.
o The right lymphatic duct:
• Receives lymph from the upper right side of the body
• It drains lymph into venous blood via the right subclavian vein.
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Lymph Trunks and Ducts
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Lymph Trunks and Ducts
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Formation and Flow of Lymph
o Lymph: The excess filtered fluid (about 3 litres per day)
in interstitial space that drains into lymphatic vessels
o Components: Similar to interstitial fluid
o Sequence of Lymph flow:
blood capillaries (blood)→interstitial spaces (interstitial fluid)
→lymphatic capillaries (lymph) →lymphatic vessels (lymph)
→lymphatic ducts (lymph) →junction of the internal jugular and
subclavian veins (blood)
o Pumps: Respiratory and muscular pumps promote flow
of lymphatic fluid
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Formation and Flow of Lymph
Fluid and proteins escaping from vascular capillaries is collected by
lymphatic capillaries and returned to the blood
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Lymphatic organs and tissues
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Lymphatic Organs and Tissues
o Widely distributed throughout the body
o Primary lymphatic organs
• provide environment for stem cells to divide and mature
into B and T lymphocytes
 Red bone marrow gives rise to mature B cells
 Thymus is the site where pre-T cells from red marrow
mature
o Secondary lymphatic organs and tissues
• site where most immune responses occur
 Lymph nodes
 Spleen
 Lymphatic nodules/follicles
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Red Bone
Marrow
o Red bone marrow:
o Location:
• In flat bones and the epiphyses of long bones of
adults
o Function:
• Site for Pluripotent stem cells differentiation: in red
bone marrow stem cells give rise to mature, immunocompetent B cells and to pre-T cells
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Thymus Gland
o Thymus Gland:
o Location:
• Between the sternum and the aorta
o Structure:
• Large organ in infants (70g) but atrophied in old age (3g)
• 2 lobed organ: Capsule and trabeculae divide it into lobules
• Each Lobules has Cortex and Medulla
o Function:
• Cortex: proliferation and maturation of Immature T cells (pre-T
cells) migrated from red bone marrow
• Medulla: Release of mature and immunocompetent T cells into
blood to migrate to lymph nodes and other lymph organs
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Thymus Gland
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Thymus Gland
o Cortex: consists of large numbers of T cells and scattered dendritic cells,
epithelial cells, and macrophages.
• Immature T cells (pre-T cells): migrate from red bone marrow to the
cortex of the thymus. They proliferate and begin to mature
• Dendritic cells: Are derived from monocytes assist the maturation
process
• Epithelial cells: produces thymic hormones (thymosin) to aid in the
maturation of T cells.
• Thymic macrophages: help clear out the debris of dead and dying cells
o Medulla: consists of widely scattered, more mature T cells, epithelial cells,
dendritic cells, and macrophages
• Site of release of mature and immunocompetent T cells
• Hassall’s corpuscles: clusters of epithelial cells; sites of T cell death in
the medulla
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Lymph Nodes
o Lymph nodes:
o Location:
• All along lymphatic vessels
• about 600 lymph nodes, scattered throughout the body, both
superficially and deep, and usually occur in groups
• Large groups are present near the mammary glands and in the
axillae and groin.
o Structure:
• encapsulated Bean-shaped organs,
• Two parts:
– The stroma: supporting framework of connective tissue
– The parenchyma: functioning part
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Lymph Nodes
o The Stroma is constituted by:
• Capsule: Outer cover of dense connective tissue that extends
into the node
• Trabeculae: The capsular extensions that divide the node into
compartments, provide support, and provide a route for blood
vessels into the interior of a node
o The Parenchyma is divided into 2 regions:
• Cortex: outer and inner
• Medulla
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Lymph Nodes
• Outer cortex:
– Consists lymphatic nodules/follicles called a Germinal
center
– Germinal center contains B cells, Dendritic cells and
macrophages
– This is the site for B cells proliferation and
differentiation into antibody-secreting plasma cells
– Also the site for where memory B cells are presented
with an antigen by dendritic cells
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Lymph Nodes
o Inner cortex:
‒ Consists mainly of T cells and dendritic cells that
enter a lymph node from other tissues.
‒ Site for T cells proliferation when dendritic cells
present antigens to T cells. The newly formed T cells
then migrate from the lymph node to areas of the
body where there is antigenic activity.
o Medulla:
‒ contains B cells, antibody producing plasma cells that
have migrated out of the cortex into the medulla, and
macrophages
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Lymph
Nodes
Lymph flow:
One direction due to
one way valves in
afferent and efferent
vessels
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Lymph Nodes
o Function:
o Acts as a filter
•
•
•
•
Many afferent vessels : Few efferent vessels
Slow flow
Lymph flows through many lymph glands to increase exposure
Within lymph glands macrophages destroy some foreign
substances by phagocytosis and lymphocytes (B cells and T
cells) bring about the destruction of others by immune
responses.
o Site of proliferation of Plasma cells (antibody secreting B
cells) and T cells.
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Metastasis Through Lymphatic
System
o Metastasis: The spread of a disease from one part of the
body to another.
o A characteristic of malignant tumours
• cancer cells travel via blood or lymphatic system
• cells establish new tumours where they lodge
o Secondary tumour sites: can be predicted by the
direction of lymphatic flow from the primary site
o Cancerous lymph nodes are enlarged, firm and nontender
o Infected lymph nodes are enlarged but not firm and are
very tender
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Spleen
o Spleen:
o Location:
• between the stomach and diaphragm
o Structure:
• the largest single mass of lymphatic tissue
• measuring about 12 cm
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Spleen
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Spleen
o Structure:
o Hilum: Through it pass the splenic artery, splenic vein,
and efferent lymphatic vessels.
o Stroma: consists of capsule, trabeculae, fibers and
fibroblasts
o Parenchyma:
• White pulp: a lymphatic tissue (lymphocytes and macrophages)
around branches of splenic artery.
• Red pulp: consists of venous sinuses filled with blood and
splenic tissue (splenic cords)
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Spleen
o Function:
White pulp: Lymphatic tissue.
B Cells, T Cells and
macrophages perform immune
function (similar to lymph node
process’)
Red pulp: consists of venous
sinuses filled with blood and
splenic tissue. Removal of
ruptured worn out RBC.
Storage of platelets.
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Lymphatic Nodules
o Lymphatic Nodules/follicles: egg-shaped masses of lymphatic
tissue that are not surrounded by a capsule
o Location: scattered throughout connective tissue of mucous
membranes
• Also known as mucosa-associated lymphoid tissue (MALT)
o Large aggregations in specific parts:
• Peyer’s patches: In the ileum of the small intestine
• Appendix
• Tonsils: form a ring at the top of throat –
– Adenoids/pharyngeal tonsil: in the posterior wall of the
nasopharynx
– Palatine tonsils: at the posterior region of the oral cavity, one
on either side
– Lingual tonsil: at the base of the tongue
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o Immune system: Overview
o Innate Immunity: Non specific resistance
• First line of defence
• Second line of defence
–Phagocytosis
–Inflammation
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Immune system: Overview
o Immunity/resistance: is the ability to ward off damage or
disease through our defences.
o The two general types:
o Innate (nonspecific) immunity: defences that are present
at birth
o Adaptive/acquired (Specific) immunity: refers to
defences that involve specific recognition of a microbe
once it has breached the innate immunity defences.
o Susceptibility: Vulnerability or lack of resistance
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Innate Immunity: Non specific
resistance
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Innate Immunity: Non specific
resistance
o Innate resistance: Immediately
effective to a large, though a
finite number of antigens that
have been coded for in our DNA
over millennia.
o Non Specific: Reactive against
conserved features common in
many pathogens (eg LPS of
gram negative bacteria)
o Two integrated lines of defence –
• The first line defence
• The second line defence
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The First Line of Defense
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First Line of Defense: Skin and
Mucous Membranes
o The first Line of defence: The external physical and
chemical barriers provided by the skin and mucous
membranes
o Physical barriers: intact epidermis layer of the skin,
mucous membranes and the lacrimal apparatus.
o Chemical barriers: is localized on the skin, in loose
connective tissue, stomach, and vagina.
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Skin and Mucous Membranes:
Physical barriers
o Epidermis: closely packed, keratinized cells, the outer
epithelial layer of the skin
o Physical barrier to the entrance of microbes
o Periodic shedding of epidermal cells helps remove
microbes
o Mucous membranes: line body cavities, trap and filter
microbes and foreign substances through
• viscous mucus
• mucus-coated hairs in nose
• cilia in upper respiratory tract
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Skin and Mucous Membranes:
Physical barriers
o Lacrimal apparatus of the eyes: manufactures and drains
away tears in response to irritants
• Protects the surface of the eyeball
• Dilute microbes and keep them from settling on the
surface of the eye.
o Saliva: Produced by the salivary glands
• Washes microbes from the surfaces of the teeth and
from the mucous membrane of the mouth,
• Reduces colonization of the mouth by microbes.
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Skin and Mucous Membranes:
Physical barriers
o Perspiration: flush microbes from the surface of the skin
o Defecation and vomiting: expel microbes through
• Vigorous contraction of the smooth muscle of the
lower gastrointestinal tract in response to some
microbial toxins
o Urine: Cleansing of the urethra to retard microbial
colonization of the urinary system
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Skin and Mucous Membranes:
Chemical barriers
o Sebum: An oily substance secreted by Sebaceous
glands of the skin
• The unsaturated fatty acids in sebum inhibit the
growth of certain pathogenic bacteria and fungi
• Forms a protective acidic film over the surface of the
skin (pH 3–5).
o Lysozyme: is an enzyme component of tears, saliva,
perspiration, nasal secretions, and tissue fluids
• Breaks down the cell walls of certain bacteria.
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Skin and Mucous Membranes:
Chemical barriers
o Gastric juice: Highly acidic (pH 1.2–3.0), produced by the
glands of the stomach
• destroys many bacteria and most bacterial toxins,
renders the stomach nearly sterile.
o Vaginal secretions:
• Slightly acidic, which discourages bacterial growth.
• Move microbes out of the body in females.
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Readings and Resources
o Tortora, GJ & Derrickson, B 2014. Principles of Anatomy
and Physiology, 14th edn, Wiley.
o Harris, P, Nagy, S & Vardaxis, N 2010, Mosby’s
Dictionary of Medicine, Nursing and Health Professions,
2nd edn, Mosby Elsevier.
o Guyton, AC & Hall, JE 2011, Textbook of Medical
Physiology, 12th edn, Saunders Elsevier.
o Marieb, EN & Hoehn, K 2010, Human Anatomy and
Physiology, 8th edn, Benjamin Cummings Pearson.
o Moore, KL, Dalley, AF & Agur, AMR 2010, Clinically
Orientated Anatomy, 6th edn, Lippincott, Williams &
Wilkins.
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