Immuno . Lec 11

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Sunday, 17/7/2011

Organs & Tissues of

the Immune System

11

25

Ziad Al-Nasser

Samah Abu-Omar, Samah Abu-Ghannam,

Dua'a Herzallah, Haya Mesmar, Basma Deeb

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Announcement about the first exam of Immunology Course:

It will be held on Saturday, 23rd of July 2011 at 8:15 a.m. You can check the Student Services on "just.edu.jo" website for more

information about the lab and your PC number.

Please, be restricted to your lab and your PC number. The exam will have 50 questions. The material included: from the beginning of chapter 1 till the end of

chapter 15.

We will have a different exam from dentistry students, because of thetime difference. Dr. Ziad Al-Nasser will write our questions and Dr.

Ammar for the dentistry students.

Organs & Tissues of the Immune System

Previously, we said that the immune system is comprised ofPrimary lymphoid

organs and Secondary lymphoid organs.

The primary lymphoid organs include the bone marrow and the thymus

gland. The bone marrow is the origin of ALL the hematopoietic cells (the major

hematopoietic organ in humans). Also, we talked about hematopoiesis and

stem cells, and how stem cells differentiate into progenitor cells then into

lymphoid cells series, myeloid cells series, erythroid series, and platelets

series. Plus, we mentioned how cytokines play a major role in thedifferentiation. So, the bone marrow is the main factory for generating the

lymphoid, myeloid, and erythroid progenitors. It is full of fat, trabeculae and

compartments. B cells develop in the bone marrow; they come from the

external part to the internal part, then mature B lymphocytes will go into the

circulation to the secondary lymphoid organs.

In the thymus gland, the T cells will develop and will be trained and will

recognize self from non-self. Then, they'll go into the circulation to the

secondary lymphoid organs.

Immunology Lecture #11Sunday, 17/7/2011

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The secondary lymphoid organs include the spleen (which is the major one),

the lymph nodes (ALL over the body), and mucosa-associated lymphoid tissue

(MALT) lining the respiratory tract, the GI tract (gut-associated lymphoid

tissue GALT; like Peyer's patch and mesenteric lymph nodes), and the

urogenital tract.

The secondary lymphoid organs are interconnected by lymphatic vessels

which pour the lymph into a major vessel just beside the inferior vena cava

(along the line of the inferior vena cava) called the thoracic duct. The thoracic

duct pours ALL of its contents into the left subclavian vein. So, ALL the

contents that come from the secondary lymphoid organs will be poured into

the blood and will be circulated.

The secondary lymphoid organs (spleen and lymph nodes) are shaped and

grouped into compartments. If they get enlarged, they become painful; which

usually indicates an inflammation. However, if they get enlarged without pain,

we become afraid that it might be a malignant transformation. Usually they

are not palpable; if they are palpable, then there is lymphadenopathy

(enlargement of lymph nodes) or splenomegaly (enlargement of spleen).

The function of the secondary lymphoid organs is to get the lymphoid cells

(that come from primary lymphoid organs) ready to act against foreign

antigens; we call that positive selection. Whereas -in the primary lymphoid

organs- lymphocytes that target self tissues will be eliminated or deleted; we

call that negative selection. So, ALL the positively selected cells are now

present in the secondary lymphoid organs and are ready to meet their

antigenic counterparts. We have macrophages and antigen-presenting cells

where the interaction takes place. The positively selected cells will change into

effector cells; whether those are B cells or plasma cells or T helper cells

(producing lymphokines or cytokines) or T cytotoxic cells which will kill virally

infected cells or tumor cells.

The primary lymphoid organs in the human embryo are initially the yolk sac,

then the fetal liver and spleen, and finally the bone marrow and the thymus.

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The Thymus Gland

If you are born without a thymus gland, you are not going to have T cells (T

from Thymus). The thymus gland (second major primary lymphoid organ) is a

bilobed organ located in the anterior mediastinum. It covers the heart, so ifyou do an operation; like open heart surgery for babies with congenital

abnormalities, you could take it out (remove it); if you take it out, NOTHING is

going to happen! Because when the baby is already born, usually the thymus

gland has fulfilled its function. The thymus continues to grow between birth

and puberty and then begins to atrophy (disappear or disintegrate), this is

calledThymic Involution.

So, we should know what the consequences are if we remove the thymus

gland at birth >> nothing is going to happen! *Very important to remember

that!* But if you are born without a thymus gland (like in DiGeorge

Syndrome), then first you are going to have severe cell-mediated immunity

deficiency; and after that, you are going to have humoral immunity deficiency.

Why is this sequence (cell-mediated immunity deficiency THEN humoral

immunity deficiency)?

Because T cells can differentiate into T helper (T helper 1, T helper 2)which will help both; the T and the B cells together. (Though some of

the B cells could be activated without the help of T helper, and this is

an exception; we'll talk about those later).

The thymus gland develops from the third pharyngeal pouch, where the

parathyroid gland develops as well. The parathyroid gland is responsible for

the production of the parathyroid hormone which functions in regulating

calcium metabolism. So, if the patient is born without the parathyroid gland,

then the patient is going to develop tetanic. So, both tetanic and cell-

mediated immunity deficiency are going to be the main land marks of

DiGeroge Syndrome [in this syndrome, there's no development of the third

pharyngeal pouch].

Important to know this relationship:

Thymus gland and Parathyroid gland develop from the third pharyngeal

pouchif the patient does not have third pharyngeal pouch

development in embryology the patient will have DiGeorge Syndromerepresented by cell-mediated immunity deficiency and tetanic.
http://en.wikipedia.org/wiki/Atrophyhttp://en.wikipedia.org/wiki/Thymic_involutionhttp://en.wikipedia.org/wiki/Thymic_involutionhttp://en.wikipedia.org/wiki/Thymic_involutionhttp://en.wikipedia.org/wiki/Atrophy

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Microscopically, in the thymus there are three main areas:

(Refer to figure 13.4)

1) Subcapsular Zone: here the earliest progenitor cells (thymocytes) will startto develop.

2) The Cortex: here there are developing T cells undergoing selection(positive or negative); so any T cell that develops a receptor for a self

antigen is going to be deleted.

How are self antigens going to be recognized in the thymus gland?

By being exposed to MHC antigens; class I and class 2.

3)

The Medulla: here you have the mature positively selected cells (the onesthat recognize foreign antigens and cannot react against self antigens).

So, the cells go from the subcapsular zone to the cortex then to the medulla.

And from there, they will go through the lymph vessels to the secondary

lymphoid organs.

So, the thymus gland is the primary site of T cell development; without the

thymus gland, T cells will never be able to develop.

95% of T cell progenitors die in the thymus gland; IMAGINE!! Which means

ALL what we are getting out of the thymus gland; ALL what we have right now,

is just 5%!

Why is that?

Those are the ones that react with self antigens! 95% of the thymocytes that

develop in the thymus gland recognize self antigens, so they have to be

negatively selected in the thymus. And in the medulla, you will see that we

have macrophages as well to get rid of those, and they also commit suicide in

a process called apoptosis! So, what we will be ending up with -in the

secondary lymphoid organs- is just the 5% of the T lymphocytes (T helper, T

cytotoxic).

In the thymus gland, you can also see stromal cells (part of the connective

tissue); these stromal cells will produce hormones that are essential in the

development and growth of the T lymphocytes. Thymopoietin is an example,

and we have so many other hormones that help nourish the growth of

thymocytes to develop into T cells in the thymus gland.

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The Spleen

Refer to figure 13.5

The spleen is the largest secondary lymphoid organ. It is located under the left

costal margin and it is not palpable. If you look at the spleen, you can see ared area and a white area, so called the red pulp and the white pulp,

respectively.

The red pulp is where the old or senile RBCs get destroyed in the spleen, while

the white pulp is where we have T & B lymphocytes stored in the spleen; and

those are present in 50%:50% according to the area.

If you look carefully into the white pulp area, the lymphocytes are present in

an area called periarteriolar lymphoid sheath; that means they are aroundblood vessels "peri-arteriolar". And around that sheath, it is ALL T cells. But in

the middle, we have islands called germinal follicles (germinal centers) where

the B cell area is located.

So, the periarteriolar lymphoid sheath (PALS) is full of T cells, while the

lymphoid follicles (germinal centers) are full of B cells.

The area between the red pulp and the white pulp is called the marginal zone;

it's a mixture of both, but mainly we here have macrophages.

We have primary and secondary germinal centers; primary for the primary

response, and secondary for the second attack (the secondary immune

response).

It is so interesting to know that we have a T cell area & a B cell area in the

secondary lymphoid organs; because if you have an immune deficiency and

you want to see in which cell line is the actual deficiency, you have to look in

the B cell area and the T cell area. If you find no follicles in the B area, it meansthe patient is going to have B cell deficiency and so antibody deficiency; the

same thing related to the T cell area.

What happens if we remove the spleen (splenectomy)?

Usually the other secondary lymphoid organs are supposed to take over and

do the job of the spleen. But you should know that the spleen plays a major

role in the defense against encapsulated bacteria; like Strep. pneumoniae & H.

influenzae.

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If a splenectomized patient gets infected with these encapsulated bacteria,

he's more likely to have a severe course of disease than those who are non-

splenectomized. So, people with splenectomy should be vaccinated against

encapsulated bacteria; they should take conjugated vaccines of Strep.

pneumo & H. influenzae in particular.

Response to polysaccharide antigens (TI: Thymus Independent) may occur in

the spleen; thats why we need to vaccinate splenectomized people against

these microorganisms.

25% of our lymphocytes are stored in the spleen. You remember, when we

wanted to make monoclonal antibodies, we took out the spleen of the mouse.

Lymph NodesRefer to figure 13.6

Lymph nodes are the most common secondary lymphoid organs that are

distributed ALL over our body. They are present in compartments

(submandibular, axillary, periaotic, mesenteric ). They are not palpable, but

you should learn how to palpate them in case of lymphadenopathy.

A lymph node is bean-shaped, less than 1 cm. We have afferent lymph vessels

(getting in) and efferent lymph vessels (getting out), and we have an arteryand a vein. We also have a specialized vein called high endothelial venule

HEV; this venule brings the lymphocytes into the assigned area in the lymph

node; whether it is a B cell area or a T cell area. It also has specialized

receptors for binding lymphocytes decreasing their motility, holding them up,

and then the lymphocytes will pass through (extravasate) the endothelium of

that vessel to its assigned area (T or B area).

A lymph node consists of a capsule, a cortical area (cortex) which is the B cell

area (primary and secondary germinal follicles), and a paracortical area

(paracortex) which is the T cell area (full of T lymphocytes).

The inner part of the lymph node is called the medulla; which has B cells, T

cells, and macrophages. Also, here is where the antigen-presenting cells are

supposed to be present; so the antigen will be taken by the APC and

presented to T helper cells; so T helper will be activated producing cytokines

that will affect the B cells which get differentiated and change into plasma

cells producing antibodies and some will develop into memory cells; ALL this

occurs here in the medulla!

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If there's no antigen, then the B cells and T cells will go through the efferent

veins into the lymph vessels then into the thoracic duct to the left subclavian

vein, and they circulate (homing or trafficking). They keep circulating among

the secondary lymphoid organs *hopefully* they will meet their antigenic

counterpart! They do around 2 rounds per day; if they couldnt find their

counterpart, then they will die :( .

So, remember The Lymph Node (slide #14):1- Its bean-shaped, and how they are arranged in groups; and you should

remember these groups and where they are located.

2- Their function as secondary lymphoid organs: storage and interaction(lymphocytes and antigens).

3- Its structure: capsule, cortical area, paracortical area, medulla, afferent& efferent vessels, the B-cell area (germinal follicle), the T-cell area

(paracortical area), and both plus macrophages in the medulla.

4- Remember the high endothelial venules (HEVs): specialized venulesthat have receptors for the lymphocytes to catch. Then, these

lymphocytes will pass through the endothelial cells into the assigned

area (B or T-cell area).

5- When they are enlarged, we call that lymphadenopathy; so when yousee patients having lymphadenopathy and splenomegaly, it means that

secondary lymphoid organs are enlarged, so you have either infection

or malignancy.

Mucosa-Associated Lymphoid Tissue (Slide #15)

Refer to figure 13.7The Mucosa-Associated Lymphoid Organs are secondary lymphoid organs as

well having B & T cells that are present over there and are supposed to meet

their antigenic counterpart.

They are found in the respiratory tract, GIT, NALT (tonsils, adenoids). [NALT:

nasopharyngeal-associated lymphoid tissue].

In the GIT, we have specialized cells called the M cells; when the antigens

encounter the M-cells, they will be taken inside, and underneath you can see

the T & B cells areas [of the gut-associated lymphoid tissue GALT], and if you

look carefully into the mucosal lining from inside to outside, you have a

specialized receptor called polyimmunoglobulin receptor which is the Scompartment of the IgA antibody.

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The IgA as a dimer is going to bind here to the receptor, and then it will be

taken inside the cell and then released into the lumen.

You can also see IgA antibodies at the respiratory epithelium as well as at the

UGT epithelium. So, the IgA antibody is going to have the S-component whichis a receptor, and the S-component is a protein that provides the IgA antibody

with protection against microorganisms.

We can sometimes do oral vaccines (subunit vaccines); for example, in the

Cholera toxin, we have two parts: A and B. The B part (binding) binds to the

mucosal surfaces and then the A part passes through the B into the cell. So, I

can vaccinate with the B component that binds with the mucosal lining so I

make protection against that particular toxin.

The intraepithelial lymphocytes that are present in the GIT play a major role

in what we call tolerance against food. The food we are eating is full of

carbohydrates and proteins [which are supposed to be highly-antigenic]; and

we dont develop reaction against the food! So, these lymphocytes that are

present have a mechanism that when you flood them with an antigen, they

will become tolerant to that particular antigen. When mechanisms of

tolerance are breached for a reason or another, then the person will develop

reaction against food (well talk about this soon).

So, food tolerance is part of our normal physiology related to our immune

system; and any breach of that would cause a reaction against food. And

remember that the main mechanism is exposing these lymphocytes to a very

large dose of an antigen, so the immune system will get paralyzed and you will

become tolerant.

The intraepithelial lymphocytes: 90% of those are T lymphocytes and 50% are

CD8+ of type; like those present in the skin. Those can move from one area

into another within the secondary lymphoid organs. They have limited

diversity of receptors and direct antigen recognition (no need for MHC

antigens). They also secrete cytokines that cause immune suppression at the

mucosa rather than immune stimulation; and we need that for the food

particles and the induction of oral tolerance.

So, oral tolerance can be caused by two mechanisms:

1- Flooding high dose of antigens2- Producing cytokines that suppress the immune system

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Remember Peyers patches and the M-cells in the GIT. Many microorganisms;

like Salmonella typhi and Brucella attack the reticuloendothelial system

(lymph nodes and lymph vessels). Sometimes, they become intracellular and

can move from one part of a secondary lymphoid organ into another.

So, when you get infected with Salmonella Typhi, the spleen will be enlarged

because this is the area where it infects. These microorganisms can go

through the M-cells and sometimes can be phagocytised by macrophages and

go through secondary lymphoid organs.

You should be familiar with infections that lead to lymphadenopathies; like

viral infections (EBV which infects B cells and we see that in the secondary

lymphoid organs as well, so you'll have splenomegaly and lymphadenopathy).

So, in Peyers patches you can see the B and T cells; mainly they are of the CD8

type, and as I said they can go from one area into another (through the

thoracic duct and bloodstream) and can provide IgA antibodies as well

through the lumen into the surface and provide protection.

The Skin (Slide #19)


The skin is the largest lymphoid organ in our body which provides us withprotection as a physical barrier in particular. Within the skin, we have antigen

presenting cells; dendritic cells and macrophages. We call these antigen

presenting cells: Langerhans cells. So when, for example, we do the (PPD)

testing, these cells play a major role in antigen presentation and what we call

delayed-type hypersensitivity reaction.

The epidermis has many Langerhans cells and T cells mainly CD8+ of type.

The dermis is full of macrophages and T-cells. So, it is important when doing

the PPD testing to be intradermal rather than subcutaneous because you want

the macrophages and dendritic cells to be involved in this process. If you go

subcutaneous, you will bypass these Langerhans cells. You see how it becomes

red and hot when you get infected or injected intradermally.

Lymphocytes Recirculation (Trafficking and Homing)

Lymphocytes keep circulating among the secondary lymphoid organs; they do

2 trips per day to find there antigenic counterparts; if they do not, they will

disappear.

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So, lymphocytes recirculation (trafficking and homing) means that they go

back home to the first place they visited; twice a day via blood stream and

lymphatics.

If you look at the amount of T lymphocytes vs. B lymphocytes in the thoracicduct before they go to the blood stream, you will see that 80-85% are T cells

rather than B cells; because T lymphocytes have a dual function; helping the

cell-mediated response and the humoral immune response, whereas B cells

are 15% only.

The first lymphocytes that come from the thymus or from the bone marrow

are called virgin lymphocytes or Nave cells. They circulate till they find an

antigen otherwise they will die, so their survival depends on meeting the

antigenic counterpart; when they find these counterparts, they will interact

with them and develop memory cells which will stay for years and years; they

can be reactivated and more memory cells will develop when exposed for the

second time; this is what we call booster dosing; it is the increase in number

of memory cells, and immediately when memory cells are exposed to the

antigen, they will react and the time period between the exposure and the

reaction will be short.

When lymphocytes are homing, they have antigens on their surface; ligands

and integrins (antigens and receptors). The antigens on the surface of the

lymphocytes have receptors on the high endothelial organs, and some of

these receptors or adhesion molecules are already there; like the selectins,

while some will develop when lymphocytes are activated; like addressins.

Inflammation sometimes develops these receptors on the endothelial cell

lining; when you are exposed to cytokines, they bind to these lymphocytes

and let them stop rolling, and then pass into the assigned area of the lymph

node for example.

So, selectins, addressins, and integrins are the names of the antigens that will

be present on the surface of lymphocytes or those of the endothelial lining so

they can bind with ligands and their receptors.

* High endothelial venules have a major role; without them trafficking and

homing will never take place!

* Lymphocytes extravasation: lymphocytes pass through the endothelial

lining into the assigned area of the lymph node.

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So, when Nave cells are exposed, they keep circulating hopefully to meet

their antigenic counterpart; and when they do, they will react. Here, we call

them prime; that means they got reacted.

Back to the receptors we talked about: (Figure 13.10)

The Doctor read the WHOLE table and commented the following:

Regarding MAdCAM-1 and GlyCAM-1: this is the homing and the arrest phase

(stoppage) of the lymphocytes at the endothelial level by MAdCAM-1 and

GlyCAM-1.

The function of "leukocyte function associated antigens": secondary

adhesion; which means that they will develop once the cells get activated.

Their ligands are the intracellular adhesion molecules; like ICAM-1.

Very late antigen 4: develop once the cells get activated.

Function of the vascular cell adhesion molecule: found on endothelial cellsthat have been activated by an inflammatory response; so when you see an

inflammation, then the VLA-4 will start to develop and will bind to CD106, and

here we have the same idea which is to hold those cells and let them pass

through the endothelium.

The extra-vasation and the passage of these lymphocytes across these high

endothelial venules:

1-

Primary adhesion to endothelium2- Followed by lymphocytes activation

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3- Secondary adhesion (arrest)4- Transmigration and chemotaxis; chemotaxins play a role in this

migration process.


Lymphocytes are rolling, and then the antigen and its receptor (for example,

CD62L -a selectin-) will bind; the selectins will hold the reaction and slow it

down, and then the addressins and integrins will put it into complete hold (the

GlyCAM-1 and CD106 for example). When cells completely stop, they will start

passing through the endothelial lining; this is what we call extra-vasation or

diapedesis, then they pass into the area of inflammation.

How are they going to be geared into the area of inflammation? Bychemotactic factors produced from the T helper cells or the tissue. They can

be geared into the area of inflammation or the assigned area in the secondary

lymphoid organs; paracortical area or germinal follicles where they will meet

their antigenic counterparts.

So, this is what we call homing or trafficking. The idea of cell homing is to look

for its antigenic counterpart, and it does that in the secondary lymphoid

organs; if it succeeds, it will continue changing into effector cells and memory

cells; if it doesn't, it will die!

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This lecture was done by the efforts of the amazing girls:

Basma Deeb, Dua'a Herzallah, Samah Abu-Ghannam, Haya Mesmar, Samah

Emad Abu Omar.

THANK YOUPEACE BE UPON THE WORLD

...

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Immuno . Lec 11