Lymphoid Tissue

Question 1

What do the V, D, J and C regions stand for

Answer 1

• V = variable
• D = diveristy
• J = joining genes
• C = constant

Question 2

What are the primary lymphoid organs?

Answer 2

• Sites where lymphopoiesis occurs
• Thymus
• Bone Marrow
• Foetal liver

Question 3

What hallmarks are characteristic of the adaptive immune response?

Answer 3

• Specificity - Vast range of B and T cell receptors
• Memory - Rapid expansion in response to secondary encounter.

Question 4

Recall the basic overview of the adaptive immune system

Answer 4

Question 5

How are the lymphoid organs structured to facilitate adaptive immunity?

Answer 5

• Haematopoiesis (specifically lymphopoiesis) increases during infection.

Question 6

What is the main site of haematopoiesis in adults?

Answer 6

The bone marrow

Question 7

Recall the differentiation of Haematopoietic stem cells into Lymphocytes including the sites where they mature.

Answer 7

Question 8

Identify the following structures and their functions

Answer 8

Question 9

Where is the B cell repertoire (range of genetically distinct BCRs or TCRS present in a given host) generated?

Answer 9

Bone Marrow

Question 10

Where do B cells undergo final maturation?

Answer 10

Periphery

Question 11

Where do immature T cells migrate to for maturation?

Answer 11

Thymus

Question 12

Where is the Thymus located?

Answer 12

above the heart

Question 13

What happens to T cells in the thymus?

Answer 13

It undergoes the following sequentially:

• Positive selection - checks if the T cell receptor can signal following interaction with the MHC and HLA molecules of the surrounding cell.
• Negative selection - does it react against our own body? Checks if the T cell is getting a large amount of signal through its T cell receptor potentially induced by a self-antigen.
• Final selection include function T cells that recognise non-self antigens and apoptosis of T-cells that don’t.

Question 14

What happens to Thymus output as you age?

Answer 14

• It declines by decreasing the output of new T cells
• Increasing adipocytes found over time

Question 15

How does a small population of T-cells find a foreign protein (antigen) in the human body?

Answer 15

Through the secondary lymphoid tissue:

• Lymph nodes
• Spleen
• Appendix
• Mucosal associated lymphoid tissue

Question 16

How do the lymphoid tissue aid in brining immune cells in close proximity to antigens?

Answer 16

• Immune tissue distributed around the body
• Interconnected via the lymphatic system and the blood
• Found as discrete organs (lymph nodes, adenoids) or distinct regions within a tissue (spleen).
• Highly organised structures
• Bring cells in close proximity to antigens.

Question 17

Key facts about Lymph Nodes

Answer 17

Question 18

Key facts about Spleen

Answer 18

• Unlike the other secondary lymphoid tissues, it’s also highly attached to the arterial circulation via the splenic artery so it can filter blood antigens rapidly and in a large number
• Not a discreet tissue on its own

Question 19

Why is the lymphoid network strong in areas surrounding the external-facing membranes of the body?

Answer 19

Lymphoid network extensive around epithelial barriers because:

• A physical barrier blocking pathogen entry
• Hence it’s the first line of defence when pathogens enter the body

Question 20

What is the gut-associated lymphoid tissue?

Answer 20

• Lymphatic drainage from the small intestinal wall drain into the payer’s patches
• They are specialised secondary lymphoid tissues called Peyer’s patches.
• Found below the epithelium of the ileum (small intestine)
• Antigens sampled by M cells
• Contain B and T cell follicles.
• Contain a high frequency of B cell germinal centres.

Question 21

What is a germinal centre

Answer 21

• It is an anatomically restricted site where B cells undergo mutation and selection to generate high-affinity antibodies.
• It’s where antibody selection occurs upon antigen encounter.
• They form in response to inflammatory antigen-rich environments.

Question 22

Why is the gut so enriched with germinal centres?

Answer 22

• It is because of the high amount of antigens found in the microbiota of the GI tract.
• Result in production of large amounts of antibodies in the GI tract to modify and modulate our microbiomes.

Question 23

Key facts about Tonsils

Answer 23

• Also external-facing secondary lymphoid tissue.
• Comprised of:
  
  • Pharyngeal
  • Tubular
  • Palatine
  • Lingual
• They encircle the oral and nasal cavity forming the Waldeyer ring.
• Specialised in antigen encounter and has its own epithelial layer where antigens can be sampled directly from the external environment.
• High frequency of lymphoid follicles in younger individuals with large germinal centres for B-cell priming.

Question 24

How do Lymphocytes and antigens get to the secondary lymphoid organs?

Answer 24

• The bodily fluid is drained between tissue cells and absorbed into the lymph with the antigen which is then taken to the lymphocytes in the SLOs.
• 2-3 litres of lymph is returned to the blood through the superior vena cava daily.

Question 25

How do rare lymphocytes with specific receptors encounter new antigens especially if they are far away from each other in the body?

Answer 25

• Naive lymphocytes do not stay within SLOs for too long
• Naive T cells recirculate once every 24 hours until they find an antigen.

Question 26

What happens after naive lymphocytes encounter antigens within the SLTs?

Answer 26

• They either will be retained and differentiated in situ within the SLTs
• Or they will migrate to the site of damage with the help of inflammatory signals.

Question 27

Describe the extravasation of lymphocytes

Answer 27

1. Rolling