Lymphoid System

Question 1

Purpose of the lymphoid system

Answer 1

Lymphatic system has 3 main roles

1) Fluid balance- preventing oedema
2) Fat absorption- from the digestive tract through lacteals which pass through lymphatic vessels into the venous circulation
3) Bodies defences- phagocytic cells and lymphocytes are found here!!! Antigen presenting cells take antigens to secondary lymphoid organs as likely that they will combine with lymphocyte with complementary receptors to antigen

Question 2

Outline lymphatic system

Answer 2

Lymphatic system consists of 2 parts:

• Network of lymphatic vessels
• Various lymphoid tissues and organs scattered all over the body

e.g. secondary lymphoid system: spleen, lymph nodes, thymus gland, tonsils, and Peyer’s patches of the intestine, as well as bits of lymphoid tissue scattered in the epithelial and connective tissues (mucosa- associated lymph tissue)

Question 3

Explain the structure of lymph follicles e.g. those frequent in digestive and airways

Answer 3

• Aggregates of thousands of lymphocytes
  Site of antigen presentation, B/T cell activation and clonal expansion
• Just under epithelia
• Kept in network of fine reticular fibres allowing APCs and lymphocytes to circulate around them
• Stretches of blood vessels with broadened epithelia for entry and exit of travelling lymphocyes

Question 4

Structure of tonsils

Answer 4

• Connective tissue (collagen) encased aggregates of follicles
• Deep in the epithelium of oral and nasal cavities forming ring around pharynx
• 5 found at back mouth, end tongue and top of nasal cavity

Question 5

Explain features within the lymph node

Answer 5

Lymph node= encapsulated aggregates of follicles, lymph percolates through them
- These act as filters interrupting the lymph vessels and flow is slow
- Surrounding the lymph node we have a fibrous capsule which forms trabeculae which divide sections of the lymph node
- Lymph enters vis many small channels at the periphery (apex)- these are the afferent lymphatic vessels
- Enters the cortex containing many follicles
-Leading onto the paracortex
Then to the medulla
Lymph flows through the medullary sinus leading to the hilum before exiting through the efferent lymphatic vessel
- Hilum side of the lymph node contains and artery and vein- called high endothelial venules
- This is important in
bringing lymphocytes from the blood into the lymph node

Question 6

Explain the structure of the lymph node and how this effects the lymphocyte distribution

Answer 6

• Cortical zone- area of B cell proliferation and maturation
• Collections of lymphocytes called primary follicles (no germinal centre) - these are called secondary follicles when a light germinal centre is seen and enlargement occurs due to division is taking place
  Paracortical zone- Darker zone containing T lymphocytes in transit (continually circulate between nodes, blood and lymph preforming surveillance role)
  As area T cell activation and proliferation- contains majority of the APC
• Medulla- high concentrations of plasma cells
• Macrophages also found here for clean up and holding indigestible material

Question 7

Explain- what is the B cell, T cell interface and why is this relevant?

Answer 7

After dendritic cell processed and presents pathogen onto MHCII carries processed peptide (antigen) to nearest lymph node

• Within cortex B cells pick up soluble antigen in lymph becoming APC
• Dendritic/ APC move to the paracortex where T cell activation and proliferation occurs
• Activated T cell and the B cell needs in the B cell- T cell interface
• Activated t helper cell will bind to the antigen presenting B cell
• Hence activating the B cell
• This will move back into the cortex to proliferate and mature- forming a germinal centre
• Some of the B cells then die off
• Majority converted into plasma and memory cells

Question 8

Why are lymph nodes clinically important?

Answer 8

Lymph nodes with high proliferative activity become enlarged and superficial lymph nodes can be palpated
- Primary tumours can metastasize (spread) from primary site, enter the lymph vessels and end up in the lymph nodes–> lymphoma
Causing enlarged non-tender lymph node
-Lymph from infected regions can drain into lymph node –> lymphadenitis- can be indication of systemic illness e.g. virus
Causing enlarged, tender lymph node

Question 9

Explain how, amongst the millions of lymphocytes the antigen Is able to find the correct antigen combining site

Answer 9

High endothelial venules- found in the paracortex mainly- consists of a thickened (cuboidal) shape
- these facilitate the constant moving of the lymphocytes between the blood and lymph system as the lymphocytes leave the blood entering lymph system through high endothelial venule

Additionally many bacteria have a few PAMPS (pathogen associated molecular pattern)

2 control systems:
Only if the T cells are activated and proliferate can an activated T cell bind to the B cell triggering B cell activation and proliferation- increases specificity of response