The Lymphatic System and immunity

Question 1

What does the lymphatic system consist of?

Answer 1

Fluid-lymph-mainly water
Vessels-lymphatics
Cells-lymphocytes (B cells, T cells and Natural Killer cells)
Supporting cells, follicular dendritic cells and macrophages
Tissues
Organs

Question 2

Where are lymphatic vessels found and how do they work?

What happens when they fail?

Answer 2

Lie adjacent to arteries and veins
Arranged into superficial and deep, flow is superficial to deep
In the limbs the deep lymphatics pass through muscles- contraction is critical to lymph circulation
Larger lymphatics eg abdomen or thorax- contraction of smooth muscle is an important contributor to lymph flow at these sites

No lymphatics in central nervous system

Failure of lymphatics leads to oedema

Question 3

Which tissues does the lymphatic system consist of ?

Answer 3

Diffuse- mucosal associated lymphatic tissue
(Includes gut associated lymphatic tissue GALT and bronchus associated lymphatic tissue BALT)

Lymphatic nodules- tonsils, peyers patches and vermiform appendix

Question 4

Which organs are associated with the lymphatic system?

Answer 4

Lymph nodes
Thymus
Spleen

Question 5

What are lymph nodes?

Answer 5

Filters for lymph
Each one has afforestation lymphatic vessels that enter via the convex surface and efferent lymphatics that leave via the hilum and a feeding artery and a draining vein that enter and leave via the hilum

Question 6

What is a follicular dendritic cell?

Answer 6

Located in the germinal centres of lymph nodes
Antigen antibody complexes adhere to dendritic processes and the cell can retain the antigen for months
This causes proliferation of B cells- especially memory B cells

Question 7

What makes an APC a professional APC?

Answer 7

B cells and macrophages are equipped with special immunostimulatory receptors that allow for enhanced activation of T cells

Question 8

Which cells mediate the inflammatory response?

Answer 8

Neutrophils and macrophages

Question 9

How do B lymphocytes develop into plasma cells and what are these?

Answer 9

B lymphocytes transform into plasma cells when stimulated by T cells. Plasma cells synthesise and secrete a specific antibody for the antigens that stimulated the T cell

Question 10

What is cell mediated immunity protecting against?

Answer 10

Viral, fungal and mycobacterium infections
Tumour cells
Transplant rejection

Question 11

What is the medical name for enlarged lymph nodes?

Answer 11

Lymphadenopathy

Question 12

What are the immune functions of the spleen?

Answer 12

Antigen presentation by APC’s
Activation and proliferation of B and T lymphocytes, production of antibodies
Removal of macro molecular antigen from blood by macrophages

Question 13

What are the haemopoeitic functions of the spleen?

Answer 13

Removal and destruction of old, damaged and abnormal erythrocytes and platelets
Retrieval of iron from erythrocytes haemoglobin

Question 14

What is the clinical importance of the spleen?

Answer 14

Very rich blood supply and relatively fragile- a ruptured spleen can easily cause death from internal bleeding.
If surgeons have to remove the spleen the liver and bone marrow take over its haemopoeitic role, however patients have slightly weakened immunity
The spleen may enlarge in response to systemic infection

Question 15

Where is and what is the thymus gland?

Answer 15

Located in the superior mediastinum

It is involved in the maturation of bone marrow derived stem cells into immunocompetent T cells (thymic cell education)

Question 16

Explain the process of phagocytosis

Answer 16

It occurs in five steps namely uptake, endosome formation, phagolysosome formation, lysis, diffusion of digested material and exocytosis. In order to uptake microorganism, pseudopodia developed arrount the microbe and takes up the microbe to form endosome.
Endosome again combines with lysosomal fraction to form phagoendosome. When the pH of the phagoendosome changed to acidic, lysosomal enzyme activated and they lyse microbes. After lysis, digested materials are passed through diffusion into cellular compoenent. Undigested materials during phagocytosis, will be excreted out. Some of these may be presented on surface for activation of T lymphocytes.

Question 17

What is oxygen dependent killing?

Answer 17

Activated phagocytes produce a number of reactive oxygen intermediates (ROIs) and reactive nitrogen intermediates that have potent antimicrobial activity. During phagocytosis, a metabolic process known as the respiratory burst occurs in activated macrophages. This process results in the activation of a membrane-bound oxidase that catalyzes the reduction of oxygen to superoxide anion, reactive oxygen intermediate that is extremely toxic to ingested microorganisms. The superoxide anion also generates other powerful oxidizing agents, including hydroxyl radicals and hydrogen peroxide. As the lysosome fuses with the phagosome, the activity of myeloperoxidase produces hypochlorite from hydrogen peroxide and chloride ions. Hypochlorite, the active agent of household bleach, is toxic to ingested microbes.

Question 18

What is opsonisation?

Answer 18

The addition of molecules to the outside of pathogens which stimulates phagocytes to ingest them

Question 19

What is complement?

Answer 19

A group of serum proteins that activates inflammation, destroys cells and participates in opsonisation
Can be activated by a number of different molecules
Respond in a sequential manner called the complement cascade
Can be activated by the classical pathway or the alternative pathway

Question 20

What is the classical pathway?

Answer 20

C1 is activated when it binds to an antigen antibody complex

Question 21

What is the alternative pathway?

Answer 21

It is activated when C3b reacts with antigens

Question 22

What is the similarities between the classical pathway and the alternative pathway?

Answer 22

Both pathways follow the same sequence after c3 resulting in inflammation, opsonisation ad membrane attack complexes

Question 23

What is oedema and lymph oedema?

Answer 23

Oedema is an accumulation of an excessive amount of watery fluid in cells due to an increase in venule pressure (can be caused by congestive heart failure due to backup of blood in vena cava) or a drop in oncotic pressure. Lymphoedema is caused by an obstruction of lymphatic vessels and does not pit.