Lymphatic System

Question 1

Define primary and secondary lymphatic tissue

Answer 1

• *Primary:** origin points of lymphocytes
• *Secondary:** site where cells undergo antigen dependent differentiation

Question 2

What are the primary lymphatic tissues?

Answer 2

• Bone marrow
• Thymus

Question 3

What are examples of secondary lymphatic tissue?

Answer 3

• Lymph Nodes
• Spleen
• Uncapsulated lyphatic tissues
  
  • MALT
  • GALT
  • Tonsils
  • Peyer’s Patches

Question 4

Cellular composition and location of the adult thymus

Answer 4

• epithelioreticular cells
• packed with stem cells
• superior mediastinum; bilobed

Question 5

Describe epithelioeticular cells

Answer 5

• Attached to each other by desmosomes
• provides supportive network for developing lymphocytes (similar to the reticular fiber network)

Question 6

Function of epithelioreticular cells

Answer 6

1. Barrier between developing T cells & antigens
2. Provides self antigens to test T cells (negative selection)

Question 7

What are the three gross anatomical areas of the thymus?

Answer 7

1. Capsule
2. Cortex
3. Medulla

Question 8

Describe the thymus capsule

Answer 8

1. Surround thymus with blood vessels, efferent lymphatic, and nerves
2. Trabeculae (capsule extensions) divides the cortex into lobules
3. hilum: where arteries, veins and lymphatic vessels exit

Question 9

Describe the thymus cortex

Answer 9

• Peripheral portion of thymus
• High density of lymphocytes
• basophilic
• Presence of macrophages & epithelial cells
• lymph follicles or nodules

Question 10

Describe the thymus medulla

Answer 10

• Central portion of thymus
• Large number of epithelioreticular cells
• Thymic (Hassall’s) corpuscles (unique to medulla)

Question 11

Thymic (Hassall’s) corpuscles

Answer 11

1. unique structures in medulla
2. Small groups of epithelial cells that are keratinized/calcified
3. May be involved in auto-reactive T-cell destruction
4. Eosiniphilic

Question 12

Function of thymus

Answer 12

• Development of immunocompetent T-cells
• Elimination of incompetent T cells

Question 13

T Cell development in the thymus

Answer 13

Arteries enter the capsule through the trabeculae of the corticomedullary junction forming a capillary bed which enters the cortex

Question 14

Three major elements of the blood-thymus barrier

Answer 14

• Capillary endothelium and is basal lamina
• Periascular connective tissue space occupied by macrophages
• Type 1 epithelioreticular cells with their basal lamina

Question 15

Function of blood-thymus barrier

Answer 15

• Creates a continuous capillary bed ensheathed by a layer of epithelioreticular cells
• Used to reduce new T-cells’ exposure to blood-borne antigens
• Creates a pathway for stem cells to develop into thymocytes in the cortex and migrate towards the medulla to mature

Question 16

How do immunocompetence cells enter circulation?

Answer 16

The venous system

Question 17

Describe thymus involution

Answer 17

Fully developed at birth & continuously makes T-cells until puberty, then becomes fat

Question 18

Where are the lymph nodes located?

Answer 18

Throughout the vascular system

Question 19

1ry follicle of thymus cortex

Answer 19

unstimulated and homogenous appearance

Question 20

2ry follicle of thymus cortex

Answer 20

• lighter staining central region (germinal center) during immune response
  
  • Immunoblasts – Large and stain light, responsible for the germinal center’s appearance
  • Mantle – Layer of mature lymphocytes around the germinal center

Question 21

Two parts of the thymus medulla

Answer 21

1. Medullary cord: dense group of cells mostly plasma cells and amino blast
2. Medullary sinuses: spaces between cord lined by macrophages

Question 22

Explain flow of lymph through the lymph node

Answer 22

• Afferent lymph vessels–>Subcapsular sinus –> Trabecular sinuses –> collect at the hilum –> leave the node through the efferent lymphatic vessels

(this exposes the lymph and antigens to a large number of lymphocytes within)

Question 23

Define subcapsular sinus of thymus

Answer 23

Space immediately adjacent to the capsule that provides a path for lymph flow

Question 24

Define trabecular sinus of thymus

Answer 24

Continual opening with subcapsular sinus that allows the lymph to flow through the cortex

Question 25

Explain blood flow and lymphocyte recirculation in a lymph node

Answer 25

Arterial supply enters through the hilum and branches into the medulla and cortex (i.e. corticalmedullary bed) --\> Loop back into the medulla and drain into the postcapillary venules --\> exits from the hilum

Question 26

What are high endothelial venules?

Answer 26

Specialized postcapillary venules in the lymph nodes lined with unusually thick endothelial cells

Question 27

What is the purpose of high endothelial venules?

Answer 27

Endothelial cells have special adhesion molecules on their luminal surface. B-cells and T-cells are able to shuttle back and forth from the lymph node to the blood. (maximizing the odds of contact with antigen)

Question 28

What are the main histological components of the spleen?

Answer 28

1. Trabeculae extend into spleen 2. Hilum 3. Parenchyma: white & red pulp

Question 29

define white pulp of the spleen

Answer 29

Round elongated accumulations of densely packed cells (lymphocytes)

Question 30

Define red pulp of spleen

Answer 30

* Surrounds white pulp * Accumulation of blood cells

Question 31

Functions of the spleen

Answer 31

* Eliminate blood-borne antigens * Destruction of old or defective blood cells (macrophages monitor blood cells by cytoplasmic extensions in the inter endothelial slits. Any difficulty of blood cell passing through is destroyed)

Question 32

Explain blood flow through the spleen and resulting filtration of defective blood cells.

Answer 32

* _Splenic artery_ * Arterial supply that enters in hilum and branches into trabecular arteries **Central artery is an arteriole branch of the trabecular arteries that area surrounded by white pulp (called the periarterial lymphatic sheath)** * * Branches from the central artery _are penicillar arterioles_ at right angles. * At the end of the penicillar arterioles _are sheath capillaries_ that are blind-ended and at the end of the _marginal zone_

Question 33

Marginal zone

Answer 33

The border of the red-white pulp border where blood passes form the sheathed capillaries and enters the splenic cords (i.e. it has exited the vasculature)

Question 34

Explain the mechanism of blood-borne antigen detection in the spleen

Answer 34

* Blood is released from the sheathed capillaries at the marginal zone --\> Ags detected by the blood are processed and presented by macrophages lining the area of the splenic sinus --\> T-cells in the spleen that recognize the Ag will be activated and leave the spleen to attack * B-cells that are activated will accumulate at the marginal zone and become plasma cells to release Abs

Question 35

Describe locations, histology, & function of major diffuse lymphoid tissues.

Answer 35

* MALT (mucosal-associated lymphoid tissues) * BALT – bronchi-associated * GALT – gut-associated * Aggregates of lymphocytes and LNs that are not completely surrounded by capsules providing a first line of defense against exogenous antigens.