Lymphoid Organs

Dr. Nabil Khouri MD, MSc, Ph.D

Learning Objectives

1. Understand the distinction between PRIMARY and

SECONDARY lymphoid organs

2. 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, T-

cells 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).

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

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

Development Other lymph organs

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

Lymphoid Tissue)

Solitary Nodules

Aggregated nodules

(Peyer’s patches)

Lymphoid nodules in

vermiform appendix

ENCAPSULATED

Direct lymphoid organs

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.

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.

LYMPHOCYTES IN CONNECTIVE TISSUE:

MALT = mucosa-associated lymphoid tissue

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

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 Antigaen-

carrying APCs and recognized

by B lymphocytes they

proliferate in the center called

“germinative Center”

This center contain follicular

dentritic cells with processes

Composition: oval concentrations of lymphocytes contained in meshwork

of reticular cells.

contain follicular

dendritic cells

Primary lymphatic

nodule/follicle (LN)

Aggregation of lymphocytes in

lamina propria or submucosa

LN

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

Germinal center: high magnification

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.

Secondary follicles/nodules

• 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 antigen-INDEPENDENT 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 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.

Peyer‟s patches

Summary

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

Tonsils

Tonsils

lymphoid organs

- 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 • 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.

Palatine Tonsils

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

The Capsule & Subcapsular sinus

Send Trabeculae & trabecular sinuses

Sinuses contain lymph, macrophages,

and reticular cells

Lymph Node Structure

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

The cortex • 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.

• 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 Sub-

capsular

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.

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

Lymphatic Circulation Through a Lymph Node

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

• 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.

The slpeen

EXTERNAL FEATURES • The spleen has two ends ,three

borders and two surfaces and 2 angles and hillum.

• TWO ENDS-

1 -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.

EXTERNAL FEATURES • Three borders-

• 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 surfaces-

1.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 structure-

1.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.

• Monitoring antigens in blood

• Proliferation of lymphocytes

• Production of humoral antibodies

• 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)

Immune Functions

Of the Spleen

Hematopoietic

Functions

Of the Spleen

The spleen clears the blood of aged blood cells and foreign particles and

is the site of immune reactions to blood-borne antigens.

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 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

The Spleen

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

RED

Pulp

WHITE

Pulp

The white 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

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

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

sinusoids

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. Red pulp

B. Venous sinus (VS) and

Cords of Billroth (BC)

C. Silver-stained section

Splenic sinuses

and cords

A B

C

Spleen (red pulp) at high power (40x)

sinus

cord

sinus

cord

U-M Histology Collection

Sinusoids

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)

Lumen of the sinusoid

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

Thymus

lymphoid organs - site of maturation of T

lymphocytes

- secretes hormones

(thymopoietin and

thymosins)

- critical role in

childhood

Thymus Gland

Location

Structure

1. Capsule and lobules

2. Cortex (T-Cell precursor, Reticuoepithelial cells, Macrophages)

3. 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.

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!

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.)