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55 Cards in this Set

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What does the lymphoid system do?
It is responsible for the immunological defense of the body

It also protects the body from foreign macromolecules, bacteria, viruses, invasive microorganisms and transformed cells.
What are the components of the lymphoid system?
The primary functional component is the lymphocytes.

Lymphocytes circulated in the blood and lymph, they are found scattered in loose connective tissue and they occur in lymphoid aggregates (clusters)

There are two types of clusters: encapsulated-lymphoid organs and in unencapsulated tissue

Have B and T Lymphocytes which serve as effector and memory cells

Have Antigen-Presenting Cells (These are macrophages, lymphoid dendritic cells, etc)
What are lymphoid organs?
Bone marrow, lymph nodes, thymus, spleen and tonsils (partially encapsulated)
What are unencapsulated aggregates?
They are lymphocyte aggregates in places with openings to the outside world.

Found in the walls of certain organ systems:
-MALT: Mucous-Associated Lymphoid Tissue
-BALT: Bronchus-Associated Lymphoid Tissue
-GALT: Gut-Associated Lymphoid Tissue
-NALT: Nasal-Associated Lymphoid Tissue
-VALT: Vulvovaginal-Associated Lymphoid Tissue
What are the four distinct properties of the immune system?
1) Specificity: Has the ability to distinguish very closely related structures

2)Diversity: Has the ability to recognize and react to a variety of structures

3) Memory: Has the ability to mount a response long after initial stimulus is gone

4) Self/Nonself Recognition: Has the ability to ignore molecules normally made by the host but to react to similar molecules from a foreign source.
How do the functional components tof the cells communicate?
Via signaling molecules
-Cytokines are released in response to the presence of antigens.
What are the two type of lymphoid organs?
--Primary )Central) Lymphoid Organs: Where B and T cells acquire immunocompetence:the ability to respond to antigenic challenges
B cells--Bone Marrow
T cells--Thymus

Secondary (peripheral) Lymphoid Organs
--Where immunocompetent cells migrate to meet antigens
THese are the lymph nodes, spleen, tonsils and diffuse aggregates
Where is the thymus located and where does it come from?
Thymus is located in the superior mediastinum.

It has mixed embryological origin:
-Lymphocytes are from Mesoderm while the Epithelial Reticular cells (form meshwork that interact with cell) are from endoderm
What is the structure of the thymus?
Has capsule:
-Made of thin, dense irregular collagenous connective tissue
-The capsule subdivides into lobes then divides into incomplete lobules

Two Lobes:
-H Shaped
-Continues to grow until puberty then begins to atrophy and fill with fat

Incomplete Lobules:
-Septa extend to separate lobes
-Have Cortex and Medulla
What image is this?
What are the different parts of this this image?
This is the thymus!

Notice the cortex and the medulla
What are the properties of the thymic cortex?
-It is the outer darker-staining region of each lobule
-Has small, densely packed, T lymphocytes suspended in meshwork of Epithelial Reticular cells

THe ERC serves to isolate T Cells from contacting antigens.

ERC has Blood-Thymus Battier

Any T cell responding to self antigens in thymic cortex is killed and phagocytized
What does the blood-thymus barrier do?
You don't want naive T cells responding inappropriately. SO the barrier acts to prevent any antigen from coming into thymus.

T cells that do well LIVE. Those that don't die which is about 80%.
What does this image show?
Thymic Cortex
What type of cells are these?
Epithelial reticular cells
What are the properties of the thymic medulla?
-Lighter staining inner region of each lobule

They have fewer cells than in cortex

Have Hassall's (Thymic) Corpuscles which are modified aged regenerating epithelial reticular cells:
-Look like swirly thing and numbers increase in age.

Cells here have proven worthy.
What image is depicted here?
What image is depicted here?
Hassall's Corpuscle
What is the function of the Thymus and what is the path that these cells take?
Their primary function is to instruct T cells

1) Immunoincompetent T cells arrive via blood system
2) Leave vascular compartment and enter the thymus at the Corticomedullary Junction
3) Migrate to the periphery of the cortex and slowly migrate to the medulla
4) Along the way they are tested with self-antigens and any positive reactions leads to death.
Are the cells in the medullar immunocompetent?
Yes but they are naive. T cells leave medulla and enter the circulation via veins draining the thymus and head to thymus organs to work.
What are the four paracrine hormones that the ER cells produce?
-Thymic Humoral Factor
What are lymph nodes?
They are structures that provide a mechanism for filetering lymph.

Help to remove bacteria and foreign debris.

Parenchyma consists of B cells, T cells, APCs, Plasma cells and Macrophages.
This is a model of the lymph node
Note the following:

Trabecula- region of tissue filled with lymphocytes/effector cells

Afferent lymph vessel that bring lymph in

Note that hilum is where the efferent vessel exit
What are the three zones of the lymph nodes?

All have a supply of sinusoids through which lymph percolates
What are the properties of cortex?
Made of dense irregular collagenous connective tissue capsule

The septa(Trabeculae) subdivide the cortex into incomplete compartments

Afferent lymphatics drain into subcapsular sinus
-This is continuous with Cortical sinuses and Medullary sinuses
- The sinuses are lined with endothelial like cells.
What type of tissue is this?
Afferent Lymphatic
What type of tissue is this?
Lymph Node
What are lymphoid nodules?
They are the accumulation of lymphocytes
What are the types of lymphoid nodules?
-Primary nodules: all the cells are the same. They are spherical aggregates of B cells.

-Secondary nodules: The site of plasma cell and memory cell formation. They form in response to an antigenic challenge. Have a light staining center nodule known as the germinal center.
What type of nodule is this?
Secondary nodule
What is the paracortex?
The region between the cortex and the medulla


Made of mostly T cells that create a thymic dependent zone

Have High Endothelial Lined Venules present.
What are the properties of the medulla?
Has a mixture of a small accumulation of cells known as medullary cords.

These cells surround a large tortuous sinuses.

The cords contain plasma cells, macrophages, and lymphocytes.

The cells are enmeshed in reticular fibers and reticular cells.

The trabeculae arising from extensions of capsule also present
What does this image depict?
The medulla of lymph node
What does this image depict?
The medulla of lymph node
What does the lymph node do?
It filters the lymph:
- SO lymph enters the node and the flow rate decreases to allow more time for interaction with resident macrophages. It is here that 99% of impurities are removed

2) Functions in Antigen Recognition and Presentation:
-Activated B cells migrate to primary nodule and begin proliferation and differentiating. This allows the secondary nodule to generate. Then differentiated plasma cells migrate to medulla. (10% stay in the medulla while 90% migrate to bone marrow)

Plasma cells are never in vessels unless cancer.
What is the spleen?
It is the largest lymphoid organ found in the left upper quadrant of the abdominal cavity
What does the spleen do?
Has two function in the adult:
1) Supports proliferation of immunocompetent B and T cells
2) Filters the blood

In some mammals it also stores blood (not in humans( and during fetal development it is ivolved in hemopoiesis (blood formation)
What is this?
the Spleen!
What is the structure of the spleen?
Has a dense, fibroelastic connective tissue capsule:

-Has occasional contractile cells
-Surrounded by visceral peritoneum made of simple squamous epithelium
-Trabeclae: Carries blood vessels into parenchyme

Has a hilum on the concave surface

Made of dense 3D network of reticular fibers.

Has venous sinuses.
What does a fresh cut spleen look like?
Has gray areas surrounded by red areas.
-The gray areas called white pulp and red areas called red pulp

In H&E section the red pulp is red and the white pulp is blue.
What is this a section of?
Study this image
Shows the circulation in the spleen
What does this image depict?
Periarterial Lymphatic Sheath
What does this image depict?
What are the properties of red pulp?
Made of splenic sinuses and cords

The sinuses are lined with fusiform endothelial cells.Each cells has a wide enough space between each other.

The splenic cords (of bilroth) are made of a loose network of reticular fibers enveloped by stellate reticular cells. The cords are red because of lots of RBCs. There are also lots of macrophages.
What is depcicted in this image?
Red pulp of spleen

The darker red are the collagen connective tissue
What are these?
Splenic sinusoid
What does the spleen do?
1) Filters the blood
2) Phagocytoses antigens, bacteria, etc.
3) Forms lymphoid cells in white pulp
4) Recycles Erythrocytes
What are tonsils and what do they do?
They are partially encapsulated aggregates of lymphoid tissue that guard the entrance to the oral cavity.
What are the 3 different types of tonsils?
Palatine Tonsils- back of the throat
Pharyngeal Tonsil- Adenoids, Mass of tissue near nose
Lingual Tonsils- base of the tongue
What is GALT?
THey are gut associated lymphoid tissue.

They are individual follicles located along the GI tract
What is Peyer's Patches?
They are a special type of GALT located only in the Ileum. Have B cells that are surrounded by T cells and APCs

They are lined by M cells.
What is depicted in this image?
Peyer's Patch
What is BALT?
THey are Bronchus Associated Lymphoid Tissue

They are found only in the respiratory tract. Lined by M cells. They have HEVs present. Mostly B cells and some T and APC present.
What is depicted in the image?