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Lymphoid Organs
Dr. Nabil Khouri MD, MSc, Ph.D
Learning Objectives
1.
2.
Understand the distinction between PRIMARY and
SECONDARY lymphoid organs
Be able to describe the general anatomical organization of:
• lymph nodes
• Spleen
• Thymus
• Mucosa-associated lymphoid tissue that include:
 Diffuse and nodular lymphoid tissue.
 regions of extensive lymphoid infiltration such as
Peyer’s patches, appendix, and tonsils.
Lymph Organs
Free lymphocytes
Mucosa-associated lymphoid tissue (MALT).
Lymph nodes
Tonsils
Thymus
Spleen
What are Primary lymphatic organs?




Primary lymphatic organs are where lymphocytes are formed
and mature. They provide an environment for stem cells to
divide and mature into B- and T- cells:
There are two primary lymphatic organs: the Red bone marrow
and the Thymus gland. The development of white blood cells
(haemopoesis) was covered briefly in the section on blood.
Both T-cell and B-cells are 'born' in the bone marrow.
However, whereas B cells also mature in the bone marrow, Tcells have to migrate to the thymus, which is where they mature
in the thymus.
What are Secondary lymphatic organs?


Secondary lymphoid tissues are arranged as a series of filters
monitoring the contents of the extracellular fluids, i.e. lymph,
tissue fluid and blood. The lymphoid tissue filtering each of
these fluids is arranged in different ways. Secondary
lymphoid tissues are also where lymphocytes are activated.
These include: lymph nodes, tonsils, spleen, Peyer's
patches and mucosa associated lymphoid tissue (MALT).
Development

LYMPH NODE DEVELOPMENT

Except for the upper portion of the cisterna chyli, which persists, the
lymph sacs are transformed into groups of lymph nodes during early fetal
life, at about month 3.

The surrounding mesenchymal cells invade each sac and break it up
into lymphatic channels or sinuses. The mesenchymal cells give rise to
the lymph node capsule and the connective tissue framework of the
node

The lymphocytes seen in the node before birth come from the thymus
gland

The lymph nodule and germinal centers of lymphocyte production do
not appear in the nodes until just before or after birth

Lymph nodes also develop along the course of other lymph vessels
Development Other lymph organs





THE SPLEEN develops from an aggregation of mesenchymal cells
in the dorsal mesentery of the stomach
THE PALATINE TONSILS form from the second pair of
pharyngeal pouches
THE TUBAL (pharyngo-tympanic) TONSILS develop from
aggregations of lymph nodules around the openings of the auditory
tubes
THE PHARYNGEAL TONSILS (adenoids) develop from an
aggregation of lymph nodules in the nasopharyngeal wall
THE LINGUAL TONSILS develop from aggregations of lymph
nodules in the root of the tongue
General Lymphoid tissue “Description”



Made up of free cells (Lymphocets, Macrophages and Plasma
cells )
And rich network of reticular fiber
 Type III collagen
 Dense - Packed with cells
 Loose - more reticular fiber fewer cells
Reticular cells produce the fibers
Two Types of lymphoid organs
NON ENCAPSULATED
(Nodular)
MALT (Mucosa Associated
ENCAPSULATED
Direct lymphoid organs
Lymphoid Tissue)



Solitary Nodules
Aggregated nodules
(Peyer’s patches)
Lymphoid nodules in
vermiform appendix




Lymph node
Spleen
Thymus
Tonsil
Mucosa-associated lymphoid tissue (MALT)
 MALT is a lymphoid connective tissue located beneath mucous
membranes in which the lymphocyte is the predominant cell type.
Occur in the respiratory, gastrointestinal, urinary and reproductive
tracts.
The exact extent of these aggregations of lymphocytes mostly B and
some T, and helper cells
They have no distinct capsule like that of lymph nodes.
MALT plays a role in regulating mucosal immunity. It may be the site
of lymphoma, usually non-Hodgkin lymphoma.
A specific entity is the MALT lymphoma linked to Helicobacter
pylori in the stomach.
LYMPHOCYTES IN CONNECTIVE TISSUE:
MALT = mucosa-associated lymphoid tissue
Diffuse lymphoid
tissue within the
Lamina propria (LP)
of gut and can be
found associated with
mucosae anywhere in
the GIT, Respiratory,
and Genitourinary
tracts.
MALT: Intraepithelial lymphocytes:
Shown here in The GIT – In which T-cells first to see antigens
Intraepithelial
lymphocytes
Shown here in resp.
epithelium.
Nodular Lymphoid Primary Tissue
Composition: oval concentrations of lymphocytes contained in meshwork
of reticular cells.





Not capsulated
Presented within the lamina
propria and mucosa of the
organs
Group of B-Lymphocytes
arrange in spheres

Called “lymphoid nodules”
(Follicles)
When activated by Antigaencarrying APCs and recognized
by B lymphocytes they
proliferate in the center called
“germinative Center”
This center contain follicular
dentritic cells with processes
LN
contain follicular
dendritic cells
Nodule — A small solid collection of tissue,
a nodule is palpable (can be felt). It may
range in size from greater than 1.0 cm (3/8
inch) to somewhat less than 2 cm (13/16
inch) in diameter. A nodule may be present
in the epidermis, dermis or subcutis
Primary lymphatic
nodule/follicle (LN)
Aggregation of lymphocytes in
lamina propria or submucosa
Germinal center: high magnification
Secondary follicles/nodules
Composition:
• Germinal center - a central region that contains large
lymphocytes, mitotic figures, macrophages, and plasma cells.
• An outer ring of small lymphocytes.
• Arise when B-lymphocytes are presented with appropriate
antigen, receive T-cell help, and then begin proliferating as
lymphoblasts
• Lymphoblasts differentiate into plasma cells or memory cells;
aberrant lymphoblasts undergo apoptosis.
• Function: morphologic indication of lymphatic tissue
response to antigen that represents a cascade of events that
includes proliferation of lymphocytes, differentiation
of plasma cell, and antibody production.
• After the antigen
presentation and
T-cell help, the
activated B-cells
set up germinal
centers in
secondary
follicles
The Appendix
Blind sac extending
from the caecum
• primary and secondary follicles
in lamina propria and
submucosa
• So, clearly a secondary
lymphoid organ…
• However, also a site of antigenINDEPENDENT differentiation
• So, also it could be considered
as primary lymphoid organ
So, associated with just
about any mucosa (GI,
respiratory, genitourinary),
you may see:
• Intraepithelial lymphocytes
(T-cells)
• Diffuse lymphoid tissue:
– B-cells
– T-cells
• Primary nodules
• Secondary nodules
– Germinal center with
lymphoblasts and
mphages
Microfold, or “M” CELLS
Are found in the gut-associated lymphoid tissue
These cells are modified intestinal epithelial cells that assist in antigen
presentation by conveying macromolecules from the intestinal lumen to
underlying compartments housing lymphocytes and macrophages.
M cells: TEM
M cells are distinguished from other intestinal epithelial cells by their
morphological differences. They are characterized by their short or no
microvilli.
When they present, the microvilli, they are short, irregular, and present on
the apical surface or pocket-like invagination on the basolateral surface of
these cells.
Distribution of MALT
• In the digestive system:
• In the wall of the pharynx - tonsils (palatine, lingual,
pharyngeal)
• In the wall of the small intestine - aggregate lymphoid
nodules (Peyer's Patches) or M cells
• In the wall of the colon-aggregate lymphoid nodules
• In the walls of the
appendix
• In the reproductive syst.
• In the wall of the vagina
Peyer patches are round or oval
and are located in the mucous
membrane lining of the intestine.
They can be seen by the naked eye as
elongated thickened areas, and their
surface is free of the projections
(villi) and depressions (Lieberkühn
glands) that characterize the
intestinal wall.
Usually there are only 30 to 40
patches in each individual. In young
adults they may be more numerous,
and as a person ages they tend to
become less prominent.
Their full function is not known, but
they do play a role in immunologic
response and contain B and T cells
similar to those found
in peripheral lymph nodes.
Peyer’s patches
• Peyer’s patches are
roughly egg-shaped
lymphatic tissue nodules
that are similar to lymph
nodes in structure, except
that they are not
surrounded by a
connective tissue capsule.
They belong to a class of
Non-Encapsulated
lymphatic tissue known as
lymphatic nodules, which
include the tonsils and
lymphatic tissue of the
appendix
Peyer’s patches
Peyer’s patches are found
throughout the ileum region
of the small intestine known
as aggregated lymphoid
nodues,
They form an important part
of the immune system by
monitoring intestinal
bacteria populations and
preventing the growth of
pathogenic bacteria in the
intestines.
Summary
Tonsils
This lymphatic tissue belong to Mucosa-associated
Lymphoid Tissue (MALT) group.
They are considered organs because they are partially
encapsulated
Tonsils are covered by an epithelium depending on their
location
They include:
1.
Palatine tonsils
2.
Pharyngeal tonsils
3.
Lingual tonsils
4.
Tubal tonsils
lymphoid organs
Tonsils
- A paired Lymphoid
structure located in the
Oropahrynx
trap and destroy bacteria
Palatine Tonsils
•The non-capsulated surface is covered contains Dense
lymphoid tissue (follicles) that forms a band of lymphatic
nodules that lie below the stratified squamous epithelium
lining the oral cavity in this region.
• Subdivided into lobes by 10-20 crypts
Palatine
Tonsils
Palatine tonsils
• Overlying epithelium forms
invaginations called
multiple crypts that
penetrate into the band of
nodules.
• These crypts act as
collecting places for
cellular debris and bacteria
as well as some living
lymphocytes that have
migrated into the crypts.
• The band of lymph nodules
is separated from
underlying tissues by a
partial capsule of dense
connective tissue.
Pharyngeal
Tonsil
 Located in the Naso-pharynx
 Covered by ciliated Pseudostratified epithelium
 In Some areas of the covering epithelium may be stratified squamous.
 Form a thin sheet of lymphoid nodules and diffuse lymphocytes
 Diffuse lymphoid tissue and nodules, but no crypts.
 Thin partial capsule of dense connective tissue separates the lymphoid tissue
from underlying tissue.
Chronic inflammation = Adenoid
Pharyngeal Tonsil
-Tonsil that has
nodules and
covered by
psedostratified
epithelium with
- No Cripts ⇒
Pharyngeal tonsil
Lingual Tonsils
They are multiple small
collections of lymphoid
tissue located at the
base of the tongue
Lingual Tonsil are
Covered by Non
keratinized stratified
squamous epithelium
One crypt for each
tonsil Or without deep
crypts ⇒ Lingual tonsil.
• Along the course of lymphatic vessels there are numerous
small Bean shaped structures called LYMPH NODES
• Usually present in groups (will be presented for you in a
separate session)
• Lymph from any part of the body passes through one or
more lymph nodes before entering the blood stream
• Lymph nodes act as filter removing bacteria and other
particulate matter from lymph
• Provides necessary microenvironment for antigen-dependent
differentiation
• Lymphocytes are
added to lymph in these
nodes
• Anatomy of lymph nodes:
•
•
•
•
•
Entire node is Bean shaped
The concavity constituting a Hilum
Usually a single lymph vessel leaves the node through its hilum.
Several lymph vessels enter the node on its convex aspect
Each lymph node consists of,
– Connective tissue framework
– Lymphocytes
Lymph Node Structure
The Capsule & Subcapsular sinus
Send Trabeculae & trabecular sinuses
Sinuses contain lymph, macrophages,
and reticular cells
The Cortex:
•An Outer “superficial” Part (B-cells)
contains:
-primary follicles/nodules
-secondary follicles/nodules
•An Inner “deep” Part (T-cells, dendritic
cells)
- The Medulla:
•medullary cords (B-cells, plasma cells)
•medullary sinuses (lymph, more
macrophages, plasma cells, and reticular
cells)
The Medulla
• The medulla of a lymph node is composed of
– medullary cords interspersed between medullary sinuses.
• The medullary cords are composed of dense lymphoid
tissue contain primary B lymphocytes their precursors
plasma cells, macrophages and T helper cells.
• The most prominent cell in the cord is the precursor to
plasma cells or immunoblasts that came from the germinal
centers of the lymphoid follicles in the cortex of the node.
• The medullary sinuses are composed primarily of
reticular fibers (RF) providing the support framework,
reticular cells (fibroblast-like cells that secret the
reticulin).
• Contain lymph , lymphocytes and macrophages
• Is composed of the cortical
sinuses surrounded by dense
accumulations of lymphocytes.
• In the more superficial cortex the
lymphocytes are arranged into
spherical follicles, lymphoid
follicles where B lymphocytes are
activated and undergo
proliferation.
The cortex
• GERMINAL CENTER (GC)
contains pale-staining cells.
• The open, pale-staining nature
of the nuclei of these cells
indicate that they are T and B
lymphocytes undergoing active
proliferation.
• Other cells include:
• Reticular cells = follicular
dendritic cells that present
antigen to the B Lymphocytes
• Macrophages that engulfed dead
B cells that have died by apotosis
Para-cortical zone, Sub-capsullar and Radial
sinuses
• Subcapsullar regions of the
cortex is made of loose lymphoid
tissue with reticular cells and
fibers
• Radial sinuses of the cortex is
placed between the nodules and
Contain primarily T
lymphocytes that do not form
into follicles.
• T lymphocytes enter the lymph
node parenchyma reside in the
Paracortical zone.
• If activated, the T lymphocytes
undergo active proliferation to
produce expanded clones of
activated T lymphocytes.
Capsule
Subcapsular
sinuses
Radial sinuses
From the sub-capsular sinus, lymph percolates through
trabecular sinuses, and finally into MEDULLARY SINUSES
High magnification view of a sinus (subcapsular sinus
shown here)
M=macrophage, Ly=lymphocytes, RF/RC=reticular fiber (and associated reticular cell)
Micrographs of lymph node of a cat showing
medullary sinuses and cords.
silver impregnation to visualize Reticular Fibers
Special stain:
•RF Form a delicate
supporting framework
for highly cellular tissues
• found in lymph nodes,
liver, bone marrow,
spleen, smooth muscle).
•Composed mainly of
Type III collagen.
•Thinner than type I
collagen
• Reticular cells. These are branched cells that contribute to the
stroma (connective tissue framework) of the lymphatic organs
and act as APCs in the thymus.
Lymphatic Circulation Through a Lymph Node
Lymph nodes filter lymph
1. Afferent lymphatic vessels drain lymph into the Subcapsular
Sinus
2. Lymph then passes to the Trabecular sinuses
3. From there, the lymph goes to the Medullary sinuses.
4. Lymphocytes and macrophages pass easily between these sinuses
and the tissue of the lymph node.
5. Macrophages in sinuses monitor the fluids. Macs phagocytose the
antigenic material and present it to T- and B-cells
Lymphocytes develop in lymph nodes (after they are formed in the bone
marrow)
T cells develop in the thymus and then enter the circulation
Macrophages and dendrite cells “present” antigen in the lymph nodes
The slpeen
• Located in the abdominal cavity, below the diaphragma, 150 gram
• The spleen lies obliquely along the long axis of the 10th rib. Thus it is Axis
is directed downwards, forward and laterally, making an angle of about
45 degrees with the horizontal plane.
EXTERNAL FEATURES
• The spleen has two ends ,three
borders and two surfaces and 2
angles and hillum.
• TWO ENDS1 -The anterior or lateral end is
expanded and is more like a
border. It is directed downwards
and forwards, and reaches the
mid-axillary line.
2-The posterior or medial end is
rounded. It is directed upwards,
backwards and medially.
• Three bordersEXTERNAL FEATURES
• 1-The superior border is charcteristically notched near the anterior
end.
• 2-The inferior border is rounded.
• 3-The intermediate border is also rounded and is directed to the
right.
• Two surfaces1.The diaphragmatic surface is convex and smooth.
2.The visceral surface is concave and irregular.
• Two Angles• 1.Anterobasal angle-It is the junction of superior border with lateral
or anterior end.
• 2.Posterobasal angle-junction of inferior border with lateral or
anterior end of spleen.
• Hilum : the hilum lies between superior and intermediate borders it
is pierced by branches and tributaries of splenic vessels.
*RELATIONS*
(a)Peritoneal relations
(b)Visceral relations
The Peritoneal relations
 The spleen surrounded by peritoneum and is suspended by
following ligaments.
 1-Gastrosplnic ligament extends from the hilum of the spleen
to the greater curvature of the stomach.
 2-Lienorenal ligament extends from the hilum of the spleen to
the anterior surface of the left kidney.
 3-phrenicocolic ligament is not attached to the spleen, but
supports its anterior end.
Four impression present are visceral surface of spleen for abdominal
organ and other structure1.Gastric impression
2.Renal impression
3.Colic impression
4.Pancreatic impression
The Hilum lies on the inferomedial part of the gastric impression
along the long axis of the spleen.
………….. Relations
Visceral Relations
Visceral surface-
Diaphragmatic surface
The surface separates the spleen from the
costodiaphragmatic recess of pleura, lung and 9th ,10th ,11th
ribs of the left side.
The spleen clears the blood of aged blood cells and foreign particles and
is the site of immune reactions to blood-borne antigens.
• Monitoring antigens in blood
• Proliferation of lymphocytes
• Production of humoral antibodies
Hematopoietic
Functions
Of the Spleen
Immune Functions
Of the Spleen
• Formation of blood cells in fetal life
• Removal and destruction of RBCs &
platelets
• Retrieval of iron from RBC hemoglobin
• Storage of RBCs and platelets (more so in
non-human species)
http://www.lab.anhb.uwa.edu.au/mb140/CorePages/Lymphoid2/lymph2.htm
The Spleen
histology
Divided into:
Red Pulp (RBC/ hemoglobin recycling)
White Pulp (responsible for immune functions)
The Spleen
The spleen general structure consists of :
Connective tissue capsule and trabeculae
Splenic pulp
Blood vessels
The connective tissue capsule consist of collagen and elastic fibres, fibroblasts
and smooth muscle cells
It sends off trabeculae into the splenic pulp
The trabeculae, carrying the larger blood vessels, branch and anastomose,
and are ultimately continuous with the branching reticular fibres and cells in
the splenic pulp
The splenic pulp involves two distinct types: WHITE AND RED PULP
Organization of the spleen: white pulp and red pulp
Only the white pulp has the character of true lymphoreticular tissue
White pulp: lymphatic aggregations around “central” arteries:
periarterial lymphatic sheath (PALS): T-cells lymph nodules: B-cells
Red pulp: is arranged in cords and sinuses
WHITE
Pulp
RED
Pulp
The white
White Pulp
The
pulp
consists of
Peri-arterial lymphatic sheath (PALS)
Lymphoid nodule surrounded by a Marginal zone
Peri-arteriolar lymphoid sheath
• The white bulb Is characterized by:
• a parenchyma that have two types of lymphocytes
• B cells and T cells located in two different areas of the spleen.
• B cells are located in the lymphoid follicle scattered throughout the organ.
• white pulp functions much in the manner that lymphoid follicles of lymph
nodes function, i.e., initiation of immune responses by B cells to foreign
antigens in the blood.
• T cells are located around the central arteries and form a kind of sheath.
• This site is called the peri-arteriolar lymphoid sheath.
The ‘White’ Pulp
• Appears basophilic on
H&E and red on silver
stain
• Site where immune
response is mounted;
formation of germinal
centers
• Germinal centers with
B cells and B cell
derivatives push the
‘central artery’ off to
the side
 The central artery is found in the white pulp
 The central artery is surrounded T cells
 Branch from the central artery into the red pulp
As the body is exposed to antigens and the immune system mounts
an immune response in the form of antibody production, lymph
nodules (w/ germinal centers) appear in the white pulp of the
spleen.
U-M Histology Collection
sinusoids
The Red Pulp
• Appears Red on H&E
• Composed of sinusoids and Cords
of Billroth
• The cords are the parenchyma of
the red pulp; they are composed
of reticular tissue w/
macrophages, red blood cells, and
lymphocytes
Cords of Billroth
Histology:
The red pulp is "red" due to
the presence of large
numbers of erythrocytes in
blood vessels called sinuses
and white pulp is "white"
due to lack of these sinuses
and consequently fewer
erythrocytes.
The red pulp surrounds the
white pulp while the latter
looks like lymphatic
nodules.
– The white pulp indicates
that there is a "central
arteriole", sometimes
called a central artery,
close to the center of each
area of white pulp.
• The red pulp of the spleen
• Characterized by a parenchyma (PN) Splenic cords supported by
reticular fiber.
• Consists of macrophages of the sheathed capillaries as well as other
macrophages and blood cells that have not yet entered the venous
sinuses.
• The rest of the red pulp is occupied by numerous venous sinuses.
• Their lining consists of long endothelial cells oriented along the
longitudinal axis of the vessel.
• Large spaces occur between adjacent endothelial cells and the
underlying basement membrane is discontinuous. Allowing blood
cells to easily pass between the endothelial cells and gain access to
the blood-stream on the venous side.
• A continuous reticular network forms the framework that supports
the macrophages and a few fibroblasts responsible for producing the
reticulin fibers; special stains are required to visualize the reticular
network.
A
B
Splenic sinuses
and cords
A. Red pulp
B. Venous sinus (VS) and
Cords of Billroth (BC)
C. Silver-stained section
C
Spleen (red pulp) at high power (40x)
sinus
cord
cord
sinus
U-M Histology Collection
Sinusoids
Lumen of the sinusoid
Lining of endothelial cells: apposed to one another, but remain
separated
Macrophages extend their processes into the lumen of the sinusoid
(you can see the remains of RBCs in macrophages)
Sinusoids
See how the basal lamina is interrupted; evident with both stains
Splenic Circulation
1. Blood enters via splenic artery at hilus
2. Splenic artery branches into trabecular arteries (which travel within
connective tissue trabeculae).
3. Trabecular arteries give off branches known as central arteries
which leave the trabecula and enter the substance of the spleen
(covered by a peri-arterial lymphatic sheath).
4. Central arteries branch into penicillar arterioles that piece through
the lymphatic sheath and spill into splenic cords.
5. Blood percolates through splenic cords and across walls of splenic
sinuses.
6. Splenic sinuses drain into pulp veins.
7. Pulp veins drain into trabecular veins.
8. Trabecular veins drain into splenic vein at the hilus.
SPLENIC CIRCULATION
Sinuses drain into splenic pulp veins, which, in turn, drain into trabecular
veins. Trabecular veins travel within trabeculae and drain into splenic vein
at the hilus.
red
pulp
white
pulp
lymphoid organs
Thymus
-
site of maturation of T
lymphocytes
secretes hormones
(thymopoietin and
thymosins)
critical role in
childhood
Thymus Gland
The Young Thymus
Surrounded by a CT capsule; cortex has a lot of
lymphocytes, fewer in the medulla
THERE ARE NO GERMINAL CENTERS IN THE
THYMUS!
Location
Structure
1.
2.
3.
Capsule and lobules
Cortex (T-Cell precursor, Reticuoepithelial cells, Macrophages)
Medulla ( T-Cells, Hassall corpuscles)
Located posterior to the
sternum in the anterior part
of the mediastinum, the
thymus is a bi-lobed nodular
organ that is very large in the
first year or two of life
reaching maximum size at
puberty then becoming
smaller in a process called
Involution.
Thymus :• In the elderly, the thymus is replaced almost entirely by
fibrous and fatty tissue and is barely distinguishable from the
surrounding tissues.
• Reticular epithelial cells secrete hormones called thymosins,
thymulin, and thymopoietin, which promote the
development and action of T cells.
• If the thymus is removed from newborn mammals, there will
be lack of immunity development.
The Thymus is a Primary Lymphoid (Immune) Organ Responsible For the
Education of T-Cells
Located over the great vessels of the heart in the area of the mediastinum
Develops from an invagination of EPITHELIUM of the 3rd pharyngeal pouch, so it is
called to be an endodermal organ.
Made of : Specialized epithelial cells (called epithio-reticular cells) that are joined to
one another by long processes with desmosomes on the extremities of the cells (like
starfish joined together at the tips) make up the bag-like support for:
Lymphocytes that, when the organ is young, fill this “bag”.
The Thymus undergoes a process called
THYMIC INVOLUTION, as T cells leave the thymus to populate other Lymphoid
effector organs, the organ shrinks, leaving only the epithelioretucular cells
NOTE: There are generally no B cells in the Thymus.
The thymus with two tissue components
• Parenchyma and Stroma.
• The parenchyma is composed mostly of T lymphocytes
in various stages of development into mature T cells
• The stroma is composed of special thymic epithelial
cells.
• The stroma consists of sparse, delicate epithelial cells
obscured by all of the lymphocytes. These epithelial
cells form the support structure for the developing T
cells but also play an important role in isolating the T
cells from foreign anitgens during their development.
Each lobule has an
outer, darker staining
cortex and an inner,
paler staining medulla.
In the cortex, (the parenchyma),
consists mostly of the
developing T lymphocytes and
reticular cells .
Epiytelial cells are called thymic
nurse cells in the cortex contain
maturing lymphocytes
In the medulla
• The stroma consists of prominent epithelial reticular
cells that have large, pale-staining nuclei and
substantial amounts of eosinophilic (pink-staining)
cytoplasm.
• There many T cells because most of them have entered
the blood stream via vessels at the cortico-medullary
junction.
• Thymic corpuscles or Hassal corpuscles
• Antigen presenting cells (APC) are also found in the
medulla where they are called Thymic interdigitating
cells.
• T cells that recognize these self-antigens Cells and are
removed by a process called apotosis.
High mag view of medulla
T-cells that survive selection process allowed to cross venule endothelium to
enter circulation.
Free lymphocytes
• can be found in the lamina propria of the
mucosa of organs of the digestive,
respiratory, urinary and reproductive
tracts.
• Plasma cells:
– Are derived from B lymphocytes that
left the blood stream in connective
tissue.
– Usually the round to oval nucleus is
eccentrically located in the cell due to
the presence of a large Golgi
apparatus. The predominant staining
pattern of the cytoplasm is bluish to
purple (basophilic) due to the large
amount of rough endoplasmic
reticulum , ribosomes. And packed
with rough ER.
– The nucleus has the appearance of
being "spoked" or having a "clock
face".
Hassall’s corpuscles
Type VI ERCs; function not very well known, but produce interleukins
(such as IL-4 and IL-7) and so likely influence T-cell differentiation
Source Undetermined
Tertiary lymphoid organs/tissues • Ectopic or tertiary lymphoid tissues develop at sites of
inflammation or infection in peripheral, non-lymphoid organs.
• These tissues are architecturally similar to conventional
secondary lymphoid organs, with separated B and T cell areas,
specialized populations of dendritic cells, well-differentiated
stromal cells and high endothelial venules.
• Most important of these sites are those tissues with direct
contact with the “external” environment, primarily the skin and
mucosal lining of the gastrointestinal, pulmonary, and
genitourinary tracts.
PRIMARY IMMUNE RESPONSE
•B-lymphocyte – antigen contact induces mitosis
(plasma cells) for more antibody carrying cells.
Antibodies released to circulatory systems.
SECONDARY IMMUNE RESPONSE
•Some “activated B-lymphocytes” become plasma
cells.
•Some remain smaller, but retain antigen-recognition
ability. (B memory cells)
•Next time similar antigenis encountered, response
is MUCH FASTER due to resident and waiting
memory cells.
T-LYMPHOCYTES
•Do not produce antibodies.
•Function in “cell-mediated immunity.”
•“NATURAL KILLER” cells destroy viruses.
•Secrete “lymphokines” which attract phagocytic cells.
•Secrete “perforin” which eats holes in the cells membrane or viral coat of
invaders.
•“Helper T cells”:
•Induce macrophages to destroy other antigens
•STIMULATE B-LYMPHOCYTES TO PRODUCE ANTIBODIES. (Can
help hundreds of B-lymphocytes mature by releasing “B-cell growth
factor.”)
•“Suppressor T Cells” prevent overreaction of the system. (Inhibit B-lymphocye
production.)