Lymphatic and Immune Systems
(Chapter 21)
Lecture # 6: The Lymphatic System
Objectives
1- List the functions of Lymphatic System
2- Explain how lymph is formed and returned to
the blood stream.
3- Name the major types of cells of the
lymphatic system and state their functions.
4- Name and describe the types of lymphatic
tissue.
5- Describe the form and function of red bone
marrow, thymus, lymph nodes, tonsils and
spleen.
Immune System
It is not an organ system, but a
population of cells that inhabit
all of our organs and defend the
body from agents of disease.
Immune cells are specially
concentrated in a true organ
system, the Lymphatic System.
Lymphatic System
It is a true organ system
composed of a network of vessels
that penetrate nearly every tissue
of the body, and a collection of
tissues and organs that produce
immune cells.
The Lymphatic System
The Lymphatic System
1- Fluid recovery
Functions of the Lymphatic System
2- Immunity
1- Fluid recovery
3- Lipid absorption
Fluid continually filters from the blood capillaries into the tissue spaces. But the blood
capillaries reabsorb only 85%.
15% (2 – 4 L/day) of the water and about half of the plasma proteins are not absorbed by
capillaries.
One function of the lymphatic system is to reabsorb this excess and to return it to the
blood.
13
mm
Hg
7
mm
Hg
2- Immunity
The region of overlap of endothelial cells acts as a one-way valve, permitting the entry of
fluid and solutes (even those as large proteins), as well as viruses, bacteria, and cell
debris, but preventing their return to the interstitial space.
On its way to the blood stream, the fluid passes through lymph nodes, where immune
cells stand guard against foreign matter. When they detect anything potentially harmful,
they activate a protective immune response.
Lymph Node
Structure
3- Lipid absorption
Special lymphatic vessels called lacteals in small intestine absorb
dietary lipids that are not absorbed by the blood capillaries.
The Lymphatic System
Lymph
Lymphatic
Vessels
It is the recovered
- Lymphatic
fluid. Lymph is
capillaries
usually a clear
colorless fluid, similar
- Lymphatic
to blood plasma but
collecting vessels
low in protein.
- Lymphatic
trunks
- Collecting ducts
Lymphoid
Tissues
Lymphoid
Organs
Aggregates of lymphocytes in the connective
tissue of mucous
membrane and
various organs.
They have well –
defined anatomical
sites and at least
partial connective
tissue capsules.
1- Diffuse lymphatic
tissue
Mucosa-associated lymphatic
tissue (MALT)
1- Red bone marrow
2- Lymphatic nodules
(follicles)
4- Tonsils
Peyer patches in the distal
portion of the small intestine
5- Spleen
2- Thymus
3- Lymph nodes
Lymph and Lymphatic Vessels
Lymph:
It is the extracellular fluid drawn into lymphatic capillaries.
It is a clear, colorless fluid, similar to plasma, but contains
much less protein.
After a meal, lymph draining from the small intestine has a
milky appearance because of its lipid contains.
Lymph leaving the lymph nodes contains a large number
of lymphocytes (indeed, this is the main supply of
lymphocytes to the blood stream).
Lymph may also contain macrophages, hormones,
bacteria, viruses, cellular debris, or even cancer cells.
Lymphatic Vessels
1- Lymphatic capillaries
(terminal lymphatics)
They penetrate nearly every tissue of the
body, but are absent from central nervous
system, cartilage, cornea, bone and bone
marrow.
They are sacs of thin endothelial cells that
loosely overlap each other closed at one
end.
The cells are attached to surrounding
tissue by protein filaments.
The gaps between cells are large enough
to allow bacteria and cells entrance to
lymphatic capillary.
The endothelium creates valve-like flaps
that open when interstitial fluid pressure is
high, and close when it is low.
Anchoring
filaments
Tissue fluid
Endothelium
of lymphatic
capillary
The lymphatic capillaries converge to form
collecting vessels.
The collecting vessels travel alongside
veins and arteries, and at irregular
intervals they empty into lymph nodes.
In the lymph nodes, bacteria are
phagocytized and immune cells
monitor the fluid for foreign antigens.
The collecting vessels converge to
form lymphatic trunks (6).
There are six lymphatic trunks, whose
names indicates their locations and part of
the body they drain.
1- Jugular trunks, 2- Subclavian trunks, 3Bronchomediastinal trunks, 4-Inter-costal
trunks, 5- Intestinal trunk, and 6- Lumbar
trunks.
Lymphatic
system
Subclavian
vein
Cardiovascular
system
Pulmonary
circuit
Collecting
ducts (2)
Lymphatic
trunks (6)
Superior
vena cava
Collecting
vessels
Collecting
vessels
Blood
flow
Lymphatic
capillaries
The lymphatic trunks converge to form
collecting ducts:
1- Right lymphatic duct
2- Thoracic duct
Lymphatic
capillaries
Systemic
circuit
Lymph
Lymph flows
forward through
open valves
Closed valves
prevent backflow
Right jugular trunk
Left jugular trunk
Right subclavian trunk
Left subclavian trunk
Right lymphatic
duct
Left subclavian
vein
Right
subclavian vein
Left
bronchomediastinal
trunk
Right
bronchomediastinal
trunk
Thoracic duct
Cisterna chyli
Intestinal trunk
Right lumbar trunk
Left lumbar trunk
The right lymphatic duct receives lymph
from right arm, right side of head and thorax;
empties into right subclavian vein.
The thoracic duct (larger and longer)
receives lymph from below diaphragm, left
arm, left side of head, neck, and thorax;
empties into left subclavian vein.
Lymphatic Cells
1- Natural killer (NK) cells
2- T lymphocytes (T cells)
3- B lymphocytes (B cells)
4- Macrophages
Lymphocytes
5- Dendritic cells
6- Reticular cells
1-Natural killer (NK) cells
They are large lymphocytes that attack and destroy bacteria, transplanted tissue, and host
cells infected with viruses or that have turned cancerous. They are responsible for
immune surveillance.
2- T lymphocytes (T cells)
T cells are born in the red bone marrow. They
are released into the blood as stillundifferentiated stem cells.
Red bone
marrow
T stem
cell
Thymus
The thymus is the “school” where they mature
into fully functional T lymphocytes.
T lymphocyte
B stem
cell
3- B lymphocytes (B cells)
B cells are also born in the red bone marrow, but they remain there until
they differentiate in functional B lymphocytes.
B lymphocyte
4- Macrophages
They are very large, avidly phagocytic cells of the connective tissue. They develop from
monocytes that have emigrated from the blood stream.
They phagocytize tissue debris, dead neutrophils, bacteria, and other foreign matter.
They process foreign matter and display antigenic fragments to certain T cells, thus
alerting the immune system to the presence of the enemy (they are antigen presenting
cells (APCs).
5- Dendritic cells
They alert immune system to pathogens that have breached
their surface.
They are branched, mobile antigen presenting cells (APCs)
found in epidermis, mucous membranes, and lymphatic
organs. In the skin they are called Langerhans’ cells.
They engulf foreign matter by receptor mediated endocytosis
rather than phagocytosis.
6- Reticular cells
They are branched stationary epithelial cells that contribute
to the stroma of a lymphatic organ. They act as APCs in the
thymus.
Lymphatic Tissues
Lymphatic (lymphoid) tissues
1-Diffuse lymphatic tissue
2-Lymphatic nodules (follicles)
They are aggregations of lymphocytes in the connective tissues of mucous membranes
and various organs. They have no fibrous capsule surrounding them.
1- Diffuse lymphatic tissue
It is the simplest form in which lymphocytes are scattered, rather than densely clustered.
It is prevalent in the mucosa of body passages open to the exterior respiratory, digestive,
urinary, and reproductive tracts, where it is called mucosa-associated lymphatic tissue
(MALT)
2- Lymphatic nodules (follicles)
They are dense masses of lymphocytes and
macrophages that congregate in some places.
- Appendix
- Peyer patches: They are dense clusters in the
ileum, the distal portion of the small intestine.
Lymphatic Organs
Lymphatic organs have well-defined anatomical sites and have
connective tissue capsule that separates the lymphatic tissue from
neighboring tissues.
Lymphatic organs
Primary lymphatic organs
Red bone marrow
They are the sites where T and B cells
become immunocompetent (able to
recognize and respond to antigens).
Thymus
Secondary lymphatic organs
Lymph nodes
They are populated with lymphocytes after
these cells are mature (immunocompetent).
Tonsils
Spleen
Immunocompetency:
It is the ability to recognize and respond to antigens.
Antigen:
It is any molecule that triggers an immune response.
Red Bone Marrow
It is a soft, loosely organized,
highly
vascular
material
separated
from
osseous
tissue by endosteum of bone.
As blood cells mature, they
push their way through the
reticular and endothelial cells
to enter the sinusoids and
flow away in the blood
stream.
Functions:
1- It is involved in hemopoiesis (blood formation).
Erythrocytes (RBCs), leukocytes (WBCs), and platelets are produced in the red bone
marrow.
2- It is an important supplier of lymphocytes to the immune system.
Thymus
Thyroid
The thymus is a member of the endocrine,
lymphatic, and immune systems.
It is a bilobed organ located in superior
mediastinum between the sternum and
aortic arch.
Trachea
Thymus
The fibrous capsule gives off trabeculae
(septa) that divide the gland into several
lobules.
The lobules have cortex and medulla
populated by T lymphocytes.
Functions:
1- It houses developing lymphocytes.
2- The reticular epithelial cells
produce
signaling
molecules
(thymosin, thymopoietin, thymulin,
interleukins, and interferon) that
promote the development and action
of T lymphocytes.
Lymph Nodes
They are the most numerous lymphatic
organs (about 450 in typical young adult).
They are elongated, bean shaped structures
with an indentation called the hilum.
They are enclosed with a fibrous capsule
with trabeculae that divide the interior into
compartments.
The parenchyma is divided into cortex and
medulla.
The cortex contains germinal centers where
B cells multiply and differentiate into plasma
cells.
Several afferent lymphatic vessels lead into
the node along its convex surface.
Lymph leaves the node through one to
three efferent lymphatic vessels that leave
the hilum.
Cervical lymph nodes
Lymph nodes are widespread but specially
concentrated in some
locations.
R. lymphatic duct
Thymus
Thoracic duct
Cisterna chyli
Abdominal,
intestinal,
and mesenteric
lymph nodes
Functions:
1- They cleanse the lymph.
2- They act as a site of T
and B cell activation.
Popliteal lymph nodes
Thoracic duct
Axillary lymph node
Spleen
Inguinal lymph nodes
Tonsils
They are patches of lymphatic tissue
located
at the entrance to the
pharynx.
Each tonsil is covered with epithelium
and have deep pits (tonsillar crypts)
lined with lymphatic nodules.
Pharyngeal tonsil or
adenoids (1)
Palatine tonsil (2)
Lingual tonsil
(numerous)
Functions:
They guard against ingested or
inhaled pathogens.
Spleen
It is the body’s largest lymphatic organ.
Functions:
Sinuses filled
with erythrocytes
1- Blood production in fetus.
2- It helps to stabilize blood volume by
transferring excess plasma from the
blood to the lymph.
3- It is an “erythrocyte graveyard” ( RBC
disposal).
Lymphocytes, macrophages and small
branches of splenic artery.
4- Lymphocytes and macrophages of
white pulp monitors blood for foreign
antigens, much like the lymph nodes do
the lymph.