Transcript Skeletal System

The Lymphatic System
Chapter 20
Introduction
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The lymphatic system supports the
function of the cardiovascular and
immune systems of the body
The lymphatic system consists of two
semi-independent parts
– A network of lymphatic vessels
– Lymphoid organs scattered throughout the
body
Introduction
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The lymphatic vessels transport fluids
that have escaped from the cardiovascular system
The main components of the immune
system (lymphocytes, lymphoid tissue,
and lymphoid organs) fight infections
and confer immunity to disease
The Lymphatic System
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An elaborate system of lymphatic vessels
runs throughout the body
These vessels collect a fluid called lymph
from the loose connective tissue around
blood capillaries and carry this fluid to
the great veins at the root of the neck
The Lymphatic System
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Because lymph flows
only toward the heart,
the lymphatic vessels
form a one-way system
rather than a full
circuit
The Lymphatic System
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There are several
orders of vessels
– Lymph capillaries
– Lymphatic
collecting vessels
– Lymph nodes
– Lymph trunks
– Lymph ducts
The Lymphatic System
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Recall that all blood
capillaries are
surrounded by a loose
connective tissue that
contains tissue fluid or
interstitial fluid
The fluid arises from
blood filtered through
the capillary walls
The Lymphatic System
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Tissue fluid consists of
small molecules of
blood plasma, water,
various ions, nutrient
molecules, and
respiratory gases
The Lymphatic System
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Tissue fluid is
continuously leaving
and re-entering the
blood capillaries
For complex reasons
slightly more fluid
arises from the
arteriole end than reenters the venule end
This amounts to
about 3 liters a day
The Lymphatic System
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The lymphatic
vessels function to
collect this excess
fluid and return it
to the bloodstream
Any blockage of the
lymphatic vessels
causes the affected
body region to swell
with excess tissue
fluid resulting in
edema
The Lymphatic System
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The lymphatic vessels also perform another
related function
Blood proteins leak slowly from blood
capillaries into the surrounding tissue fluid
Lymph vessels return leaked proteins to the
bloodstream
This is important because proteins in blood
generate osmotic forces that are essential for
keeping water in the bloodstream
Lymph Capillaries
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Lymph capillaries are permeable vessels that
receive the tissue fluid
They are located near blood vessels in the loose
connective tissue
Like blood capillaries their walls consists of a single
layer of endothelial cells
Lymph Capillaries
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Lymph capillaries
are very permeable
The permeability
results from the
structure and
arrangement of the
endothelial cells
They have few
intercellular
junctions
Lymph Capillaries
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In lymph capillaries
the adjacent edges of
cells overlap to form
easily opened minivalves
These valves open to
allow tissue fluid to
enter the lymphatic
capillary
Collagen filaments
anchor the cells to
the connective tissue
Lymph Capillaries
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The minivalves work because the collagen
filaments anchor portions of the cell wall
to the surrounding connective tissue
Any fluid pressure in the volume of the
tissue fluid separates the minivalves
This opens gaps in the wall of the capillary
allowing fluid to enter
Once in the lymphatic capillary, it cannot
leak out, because backflow pressure forces
the minivalve flaps together
Distribution of Lymphatic Vessels
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Lymph capillaries are widespread,
occurring almost everywhere blood
capillaries occur
Lymph capillaries are absent from bone
and teeth, bone marrow, and the entire
central nervous system
Lymph Capillaries
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Once interstitial fluid enters the
lymphatic ducts it is called lymph
Although the high permeability of lymph
capillaries allows the uptake of large
quantities of tissue fluid and protein
molecules it also allows bacteria, viruses,
or cancer cells in the loose connective
tissue to enter these capillaries with ease
Lymph Capillaries
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These pathogenic agents can then travel
throughout the body via the lymphatic
vessels
However, most pathogenic agents are
destroyed in the lymph nodes by various
antibodies before reaching the general
circulation
Cancer cells can be the most problematic
and can actually use lymph nodes as a
site to metastasize further
Lymph Capillaries
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Lymph capillaries
are widespread,
occurring almost
everywhere blood
capillaries occur
Lymph Capillaries
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Lymph capillaries are absent from bone
and teeth, from bone marrow, and from
the entire CNS
In the CNS excess fluid drains into the
cerebrospinal fluid and then returns it to
the blood at the superior sagittal sinus
Lymph Collecting Vessels
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From the lymph
capillaries, lymph
enters lymphatic
collecting vessels
which accompany
blood vessels
Lymph Collecting Vessels
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In general, the
superficial
lymphatic
collecting vessels
in the skin travel
with superficial
veins
Deep lymphatic
collecting vessels
of the trunk and
digestive viscera
travel with the
deep arteries
Lymph Collecting Vessels
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Lymphatic collecting vessels are narrow
and delicate and are usually not seen in a
laboratory dissection
The vessels have the same tunics as blood
vessels but their walls are always thinner
The thinness reflects the fact that lymph
flows under very low pressure and have
no pump to move the lymph along
Lymph Collecting Vessels
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To direct the flow of
lymph, lymphatic
collecting vessels
contain more valves
than do veins
At the base of each
valve, the vessel
bulges, forming a
pocket in which
lymph collects and
forces the valve shut
Lymph Collecting Vessels
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Because of these
bulges, each
collecting vessel
resembles a string of
beads
This distinctive
appearance, which
characterizes the
larger lymph ducts as
well allow MD’s to
recognize lymph
vessels in X-rays
Lymph Collecting Vessels
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Unaided by pressure from a pump,
lymph is propelled through lymph vessels
by a series of weaker mechanisms
The action of contracting skeletal muscle
and the pulsation of nearby arteries push
on lymph vessel, squeezing lymph
through them
Lymph Collecting Vessels
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The muscular tunica media of the lymph
vessels also contacts to help propel the
lymph
Additionally, the normal movements of
the limbs and trunk keep the lymph
flowing
Despite these mechanisms, the transport
of lymph is slow
People who are inactive or who stand for
long times often develop edema
Lymph Nodes
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Lymph nodes, which
cleanse the lymph of
pathogens are bean
shaped organs situated
along lymphatic
collecting vessels
The term lymph gland is
not correct because they
are not glandular in
their function
There are about 500
lymph gland in the body
Lymph Nodes
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Large clusters of
superficial lymph nodes in
the cervical, axillary, and
inguinal regions
The superficial cervical
nodes along the jugular
and carotid arteries receive
lymph from the head and
neck
Lymph Nodes
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Axillary nodes in the
armpit filter lymph
from the upper limbs
Lymph Nodes
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The inguinal nodes in
the superior thigh
filter lymph from the
lower limb
Lymph Nodes
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Nodes in the
mediastinum such as
the deep
tracheobronchial
nodes receive lymph
from the thoracic
viscera
Lymph Nodes
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Deep nodes along the
abdominal aorta,
called aortic nodes,
filter lymph from the
posterior abdominal
wall
Lymph Nodes
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Finally, deep nodes
along the iliac
arteries, called iliac
nodes, filter lymph
from the pelvic
organs and the lower
limbs
Lymph Nodes
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The microscopic
anatomy of a
lymph node
suggests its role as
a body filter
The node is
surrounded by a
fibrous capsule of
dense connective
tissue
Lymph Nodes
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Fibrous strands of
connective tissue
called trabecule
extend inward to
divide the node
into compartments
Lymph Nodes
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Lymph enters the
convex aspect of
the node through
several afferent
lymphatic vessels
and exits from the
indented region on
the other side, the
hilus, through
efferent lymphatic
vessels
Lymph Nodes
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Between the
afferent and
efferent vessels,
lymph percolates
through lymph
sinuses
These large lymph
capillaries are
spanned internally
by a crisscrossing
network of
reticular fibers
Lymph Nodes
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The reticular fibers are
covered by star-shaped
endothelial cells
Many macrophages live
on this fiber network,
phagocytizing
pathogens and foreign
particles in the lymph
that flows through the
sinuses
Lymph Nodes
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Most lymph passes through several nodes
It is usually free of pathogens by the time
it leaves its last node and enters the
lymph trunks on its way to the great
veins in the neck
Lymph Nodes
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Along with its
lymph sinuses, a
lymph node also
contains tadpole
shaped masses of
lymphoid tissue
This tissue is
divided into outer
(cortex) and inner
(medulla) regions
and are part of the
immune system
Lymph Trunks
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After leaving the
lymph nodes, the
largest lymphatic
collecting vessels
converge to form
lymph trunks
These trunks
drain large areas
of the body
Lumbar Trunks
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The paired lumbar
trunks lie along
either side of the
aorta in the
inferior abdomen
They receive all
lymph draining
from the lower
limbs, pelvic
organs and some
of the anterior
abdominal wall
Intestinal Trunk
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The unpaired
intestinal trunk
lies near midline
on the posterior
abdominal wall
It receives fatty
lymph (chyme)
from the stomach,
intestines, and
other digestive
organs
Brachiomediastinal Trunks
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These paired
trunks ascend near
the sides of the
trachea
They collect lymph
from the thoracic
viscera and
thoracic wall
Subclavian Trunks
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These are paired
trunks located
near the sides of
the trachea
These trunks
receive lymph
from the upper
limbs
They also drain
the inferior neck
and the superior
thoracic wall
Jugular Trunks
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These trunks are
located in the neck
at the base of each
internal jugular
vein
These trunks
drain lymph from
the head and neck
Lymph Ducts
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The lymph trunks drain into the largest
vessels the lymph ducts
The number of ducts in an individual
may vary from two to only only duct
Thoracic Duct
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The thoracic duct
is present in all
individuals
Its most inferior
part, located at
the union of the
lumbar and
intestinal trucks is
the cisterna chyli
which lies on the
bodies of
vertebrae L1 + L2
Thoracic Duct
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The thoracic duct
ascends along the
vertebral bodies
In the superior
thorax, it turns
left and empties
into the venous
circulation at the
junction of the
internal jugular
and left
subclavian veins
Right lymphatic Duct
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Some people have
a short right
lymphatic duct
formed by the
union of the right
jugular,
subclavian, and
bronchomediastinal
trunks
Right lymphatic Duct
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When present, this
duct empties into
the neck veins at or
near the junction of
the right internal
jugular and
subclavian veins
The right lymphatic
duct drains the
upper quarter of
the body
Right lymphatic Duct
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In individuals
without a right
lymphatic duct,
the three trunks
empty separately
into the neck veins
Lymphatic Vessels
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In summary, the lymphatic vessels….
– Return excess tissue fluid to the bloodstream
– Return leaked proteins to the blood
– Carry absorbed fat from the intestine to the
blood through lacteals
Immune System
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The immune system is central to the
body’s fight against disease
Unlike the body’s other defense systems,
it recognizes and attacks specific foreign
molecules
It destroys pathogens more and more
effectively with each new exposure
Immune System
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The immune system centers around the
key defense cells from lymphocytes
But it also includes lymphoid tissue, and
the lymphoid organs these include…
– Lymph nodes, spleen, thymus, tonsils,
aggregated lymphoid nodules in the small
intestine, and appendix
Lymphocytes
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Infectious microorganisms that penetrate
the epithelial barriers of the body enter the
underlying loose connective tissues, where
they are attacked by the inflammatory
response, by macrophages and finally, by
lymphocytes of the immune system
Lymphocytes are white blood cells and that
each lymphocyte recognizes and attacks its
own type of foreign molecule, called an
antigen
Lymphocytes
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B lymphocytes multiply to become
plasma cells that secrete antibodies
Cytotoxic (CD8+) T lymphocytes destroy
antigen bearing cells by penetrating their
membranes and inducing programmed
cell death
Lymphocytes
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B and T cells continuously travel in the
blood and lymph streams to reach
infected connective tissues throughout the
body, where they fight infection
They repeatedly enter and exit these
connective tissues, including the often
infected lymphoid tissue, by squeezing
through the walls of capillaries and
venules
Lymphocytes
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This repeated movement of activated
lymphocytes between the circulatory
vessels and the connective tissues, called
recirculation, ensures that lymphocytes
reach all infection sites quickly
Lymphocyte Activation
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Immature lymphocytes go through
several stages before they are able to
attack antigens
Most lymphocytes pass through these
stages during infancy and childhood, by
many do so in adulthood as well
Lymphocyte Activation
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This illustration provides an overview of
lymphocyte activation
Lymphocyte Activation
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Lymphocytes
originate in the
bone marrow
from lymphoid
stem cells, some
of which travel in
the bloodstream
to the thymus in
the thorax and
become T
lymphocytes (T is
for Thymus)
Lymphocyte Activation
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Other
lymphocytes stay
in the bone
marrow and
become B
lymphcytes
Lymphocyte Activation
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These new T and B lymphocytes divide
rapidly and generate many lymphocyte
families (clones), each of which is able to
recognize one unique type of antigen (this
is called gaining immunocompetence)
Lymphocyte Activation
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Young T or B
lymphocytes
travel through
the bloodstream
to an infected
connective tissue,
where it binds to
its specific
antigen, an
encounter called
antigen challenge
Lymphocyte Activation
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As a consequence of the antigen challenge
the lymphocyte becomes fully activated,
gaining the ability to attack its antigen,
proliferates rapidly, and produces mature
lymphocytes that recirculate throughout
the body seeking pathogens to attack
Lymphocyte Activation
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During the antigen challenge, an activating
lymphocyte interacts with several other
cells types
The lymphocyte receives its antigen from
an antigen presenting cell, such as a
macrophage that has recently phagocytized
the antigen, or a star shaped dendritic cell,
a professional antigen gathered that patrols
the body seeking antigens and carries them
to places where lymphocytes gather
Lymphocyte Activation
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A distinct type of lymphocyte, called a
helper (CD4+) T lymphocyte, secretes
chemical signals that greatly stimulate
the proliferation of activating B and
cytotoxic T lymphocytes
Helper T cells are important because
their signals amplify and fine-tune the
immune response
Lymphocyte Activation
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The importance of helper T lymphocytes
is illustrated by acquired immune
deficiency syndrome (AIDS), a viral
disease in which a drastic decline in the
body’s helper T cells greatly weakens the
immune system
Lymphocyte Activation
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As more activating T or B cells
proliferates within infected connective
tissue, it produces two types of mature
lymphocytes, effector and memory
Short lived effector lymphocytes attack
the pathogen immediately and then die
Memory lymphocytes, by contrast, wait
until the body encounters their antigen
again - maybe decades later
Lymphocyte Activation
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When a memory lymphocyte finally
encounters its antigen, its proliferating
response and its attack are most vigorous
and rapid
Memory lymphocytes are the basis for
acquired immunity
They guard against subsequent infections
and prevent us from getting many
diseases more than once
There are T and B varieties of memory
lymphocytes
Lymph Nodes: Clinical
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Inflammation of a node is caused by a
large number of bacteria trapped in a
node
– Inflammation results in swelling and pain
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Lymph nodes can become secondary
cancer sites, particularly in metastasizing
cancers that enter lymphatic vessels and
become trapped
– Cancer infiltrated nodes are swollen but not
painful
Lymphoid Tissue
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Lymphoid tissue is an important
component of the immune system
because it
– Houses and provides a proliferation site for
lymphocytes
– Furnishes an ideal surveillance vantage point
for both lymphocytes and macrophages
Lymphoid Tissue
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Lymphoid tissue is the most important
tissue of the immune system
The tissue is an often infected connective
tissue in which vast quantities of
lymphocytes gather to fight invading
microorganisms
This tissue had two general locations
– Mucous membranes
– Lymphoid organs
Lymphoid Tissue
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Mucous membranes
– Found within the digestive, respiratory,
urinary and reproductive tracks where it is
called mucosa-assocaited lymphoid tissue
(MALT)
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Lymphoid Organs
– Lymph nodes, spleen, thymus, tonsils,
aggregated lymphoid nodules, and appendix
Lymphoid Tissue
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This tissue is the main battleground in
the fight against infection
Lymphoid tissue is where most
lymphocytes become activated and most
effector and memory lymphocytes are
generated
Lymphoid Tissue
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Lymphoid tissue, a
type of loose
connective tissue
called reticular
connective tissue,
dominates all
lymphoid organs
except the thymus
The dark staining
areas represent the
connective tissue
fibers
Lymphoid Tissue
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The structural features
of lymphoid tissue serves
its infection fighting role
It is a reticular
connective tissue whose
basic framework is a
network of reticular
fibers secreted by
reticular cells
(fibroblasts)
Lymphoid Tissue
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Within the spaces
of this network
reside the many T
and B lymphocytes
that arrive
continuously from
venules coursing
through the tissue
Macrophage
Lymphocytes
Reticular
fiber
Lymphoid Tissue
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Macrophages on
the fiber network
kill invading
microorganisms by
phagocytosis and
along with
dendritic cells, they
activate nearby
lymphocytes by
presenting them
with antigens
Macrophage
Lymphocytes
Reticular
fiber
Lymphoid Tissue
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Evident within
lymphoid tissues
are scattered,
spherical clusters
of densely packed
lymphocytes,
called lymphoid
nodules or follicles
These nodules
often exhibit
lighter staining
germinal centers
Lymphoid Tissue
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Nodules derive from
the activation of a
single B cell, whose
rapid proliferation
generates the
thousands of
lymphocytes in the
nodule
Newly produced B
cells migrate away
from the nodule to
become plasma cells
Lymphoid Organs
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Lymphoid organs are
the lymph nodes,
spleen, thymus,
aggregated lymphoid
nodules in the small
intestine, and
appendix
Lymphoid Organs
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Lymph nodes are
more than filters
The regions of the
node between the
lymph sinuses are
tadpole shaped
masses of lymphoid
tissue
Lymphoid Organs
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As lymph moves
through the sinuses,
some of the contained
antigens leak out
through the sinus
wall into the
lymphatic tissue
Most antigen
challenges in the
human body occur in
the lymph nodes
Lymphoid Organs
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In the lymph nodes
antigens are
destroyed and B and
T lymphocytes are
activated
The activation adds
to the body’s supply
of memory
lymphocytes that
offer long term
immunity
Lymphoid Organs
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Lymph nodes have
two histologically
distinct regions, an
external cortex and
a medulla
All the lymphoid
nodules and most B
cells occupy the
lymphoid tissue of
the most superficial
part of the cortex
Lymphoid Organs
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Deeper in the cortex
the lymphocytes are
primarily T cells,
especially helper T
cells that increase
the activity of B cells
in the nearby
nodules
Lymphoid Organs
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Thin, inward
extensions from the
cortical lymphoid
tissue help define the
medulla
These cord like
medullary
extensions contain
both T and B
lymphocytes, plus
plasma cells
Spleen
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The soft, blood rich
spleen is the largest
lymphoid organ
Its size varies
greatly among
individual, but on
average it is the
size of a fist
Lymph Nodes
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Dendritic cells nearly encapsulate the
follicles and abut the rest of the cortex,
which primarily houses T cells in transit
The T cells circulate continuously
between the blood, lymph nodes, and
lymphatic stream, performing their
surveillance role
Spleen
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The large splenic
vessels enter and
exit the spleen on
the anterior
surface along a line
called the hilus
Spleen
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The spleen has two main blood cleansing
functions
– The removal of blood-borne antigens (its
immune function)
– The removal and destruction of aged or
defective blood cells
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Additionally, the spleen is a site of
hematopoiesis in the fetus and stores
blood platelets throughout life
Spleen
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The spleen is
surrounded by a
fibrous capsule from
which trabeculae
extend inward
The larger branches
of the splenic artery
run in the trabeculae
and send smaller
arterial branches into
the substance of the
spleen
Spleen
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The arterial branches
are called central
arteries because they
are enclosed by thick
sleeves of lymphoid
tissue that collectively
constitute the white
pulp of the spleen
Spleen
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Blood borne antigens
enter this lymphoid
tissue and are
destroyed as they
activate the immune
response
Surrounding the
white pulp is red pulp
which has two parts
– Venous sinuses
– Splenic cords
Spleen
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Venous sinuses are
sinusoid capillaries
that arise from the
distal branches of the
central arteries
outside of the white
pulp
Spleen
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Splenic cords consist
of reticular
connective tissue that
is exceptionally rich
in macrophages
Whole blood leaks
from the sinuses into
this connective tissue
where macrophages
then phagocytize any
defective blood cells
Spleen
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Red pulp is responsible for the spleen’s
ability to dispose of worn-out blood cells
White pulp provides the immune
function of the spleen
Thymus
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The two lobed
thymus lies in the
anterior thorax and
inferior neck, just
posterior to the
sternum
Thymus
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The thymus is the site at which immature
lymphocytes develop into T lymphocytes
The thymus secrets thymic hormones
such as thymopoietin which causes T
lymphocytes to gain immunocompetence
Thymus
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Prominent in newborns, the thymus
continues to increase in size during
childhood when it is most active
During late adolescence, it begins to
atrophy gradually, as its functional tissue
is slowly replaced with fibrous and fatty
tissue
Thymus
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At age 20 it still has about 80% of its
functional tissue but at age 40 it typically
retains only 5% of its functional tissue
By age only 2% of functional tissue
remains and the thymus is a fatty mass
that is difficult to distinguish from
surrounding connective tissue
However, even as it atrophies, the thymus
continues to produce immunocompetent
cells throughout adulthood (reduced rate)
Thymus
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Again, the thymus lies in the superior thorax
Thymus
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The thymus contains numerous lobules arranged
like fronds in the head of a cauliflower
Each lobule contains an outer cortex and an
inner medulla
Thymus
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The cortex is packed with rapidly
dividing T lymphocytes gaining
immunocompetence
The medulla contains fewer lymphocytes
In addition, the medulla contain the
thymic (Hassall’s) corpuscles which seem
to be collections of degenerating
epithelial reticular cells
The number and size of these corpuscles
increases with age
Thymus

The thymus differs from other lymphoid
organs in two basic ways (First)
– It functions strictly in lymphocyte
maturation and thus is the only lymphoid
organ that does not directly fight antigens
– The blood-thymus barrier, keeps bloodborne antigens from leaking out of thymic
capillaries and prematurely activating the
immature thymic lymphocytes
Thymus
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The thymus differs from other lymphoid
organs in two basic ways
(Second)
– The tissue framework of the thymus is not a
true lymphoid connective tissue
– The thymus arises like a gland from the
epithelium lining the embryonic pharynx
– Its basic tissue framework consists of star
shaped epithelial cells rather that reticular
fibers
Thymus
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The thymus differs from other lymphoid
organs in two basic ways
(Second)
– These epithelial reticular cells secrete the
thymic hormones that stimulate T cells to
become immunocompetent
– The thymus has no lymphoid nodules
because it lack B cells
The Tonsils
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The tonsils are perhaps the simplest
lymphoid tissue
They are mere swellings of the mucosal
lining of the pharynx
The Tonsils
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There are four groups of tonsils, palatine,
lingual, pharyngeal, and tubal
The Tonsils
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The palatine tonsils lie directly posterior
to the mouth and palate on the lateral
sides of the pharyngeal wall
These are the largest tonsils and the ones
most often infected and removed during
childhood (tonsillectomy)
The lingual tonsil lie on the posterior
surface of the tongue
The Tonsils
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The pharyngeal tonsil (adenoids) lie on
the pharyngeal roof
The tubal tonsils are just behind the
openings of the pharyngotympanic tubes
into the pharynx
The Tonsils
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The four tonsils are arranged in a ring
around the entrance to the pharynx to
gather and remove many pathogens that
enter the pharynx in inspired air and
swallowed food
The tonsils process the antigens, then set
up immune responses
The Tonsils

The tonsils
consist of an
epithelium
underlain by a
connective tissue
lamina propria
The Tonsils

In the tonsils the
underlying propria
consists of
abundant mucosa
associated
lymphoid tissue
(MALT) packed
with lymphocytes
and scattered
lymphoid nodules
Germinal
centers
The Tonsils

The overlying
epithelium
invaginates deep
into the interior
forming blind
ended structures
called crypts that
trap bacteria and
particulate
matter
The Tonsils

The trapped
bacteria work their
way through the
epithelium into the
underlying lymphoid
tissue causing the
activation of
lymphocytes
The Tonsils


The trapping of bacteria in the crypts
leads to many infections during
childhood
But it also generates a great variety of
memory lymphocytes for long-term
immunity
The Tonsils


By inviting an
infection, the tissue
produces a wide
variety of immune
cells with a
“memory” for the
trapped pathogens
The early risk
during childhood
results in better
health in adulthood
Aggregates of Lymphoid Follicles



Many bacteria permanently inhabit the
hollow interior of the intestines and are
constantly infecting the intestinal walls
To fight these invaders, MALT is
especially abundant in the intestine
In two parts of the intestine, MALT is so
large, permanent, and densely packed
with lymphocytes that is said to form
lymphoid organs: Aggregated lymphoid
nodules and the appendix
Aggregates of Lymphoid Follicles


Aggregated
lymphoid nodules
(Peyer’s patches) are
clusters in the walls
of the distal part of
the ileum of the small
intestine
About 40 of these
patches are present
averaging about 1 cm
and 1 cm wide
Peyer’s Patches
Aggregates of Lymphoid Follicles

Aggregates of lymphoid follicles are ideally
situated to destroy bacteria thereby
preventing these pathogens from breaching
the intestinal wall
The Appendix

The appendix is a
tubular offshoot of
the first part of the
(cecum) of the large
intestine
The Appendix


Lymphoid tissue is
also heavily
concentrated on the
walls of the appendix
Histological sections
reveal that dense
lymphoid tissue
uniformly occupies
over half the
thickness of the wall
of the appendix
The Appendix

Beside destroying the microorganisms
that invade them, the aggregated
lymphoid nodules and the appendix
sample many different antigens from
within the digestive tube and generate a
wide variety of memory lymphocytes to
protect the body