Immunology Immune Cells and Organs

Question 1

How does the lymphatic system operate?

Answer 1

Fluid drained from between tissue cells absorbed into lymph
- 2 to 3 litres of lymph are returned to the blood each day (via superior vena cava)
- In the process of draining, lymph can “capture” pathogens
- Fluid passes through lymph nodes which survey for pathogens

Question 2

What is the whole process of function of lymph nodes? Movement of lymph, shape of lymph nodes, entering and exiting, names of junctions,

Answer 2

LYMPH NODES
- Kidney shaped organs > 1cm
- During immune response, swell in size
- Fluid enters through AFFERENT vessel
- Fluid leaves via EFFERENT vessel
- Lymph perculates through all lymphocytes before
leaving the node
- Usually a SUMMATIVE junction, i.e. there are many
afferent vessels but one efferent vessel
- Rich blood supply lets lymphocytes into the lymph
nodes via the HIGH ENDOLTHELIAL VENUES
- T-cell zone: parafollicular cortex
- B-cell zone: lymphoid follicle- mostly on the periphery of the lymph node
- During immune response, there is a massive proliferation of B cells, which leads to the formation of a
GERMINAL CENTRE
- Specific chemokines target their respective lymphocytes to their specific areas, e.g. T-cells to the
parafollicular cortex
- The lymph entering lymph nodes may also contain cells such as dendritic cells and macrophages

Question 3

What is the spleen used for? Process? Structure?

Answer 3

Spleen
- Filter for antigens in the blood
- Large organ in the abdomen
- Separated into
white pulp: lymphoid cells around blood vessels, full of lymphocytes
red pulp: contains old damaged RBC
- Any diseases involving RBC, i.e. sickle-cell, often results in an enlargement of the spleen

• T cell area: peri-arteriolar lymphatic sheath (PALS)
• B cell area is located further away from blood vessels
• Not a vital organ: Individuals who do not have a spleen are highly susceptible to infections with encapsulated bacteria

Question 4

What does the mucosal associated tissue do?

Answer 4

Mucosal Associated Lymphoid Tissue (MALT)
• Epithelium is the first line of defence
• mucosae and skin form a physical barrier
• very large surface area, in large part a single layer of cells
• heavily defended by the immune system in case it breaks

Question 5

Secondary Lymphoid Organs

Gut Associated Lymphoid Tissue - Structure, Function?

Answer 5

• Many villi, plus smoother regions
• Involved in the mesenteric lymphatic drainage system to mesenteric lymph nodes, including intraepithelial lymphocytes
• PEYER’S PATCH: non-capsulated aggregation
  of lymphoid tissue- predominantly B lymphocytes and contain germinal centres during immune responses
• M-CELLS: sample contents of the intestine, surveying for pathogens which they can then deliver to immune cells

Question 6

What is the function of the cutaneous immune system?

Answer 6

Cutaneous Immune System
- I.e. the skin
- Epidermis contains keratinocytes, Langerhans cells
and intraepidermal lymphocytes
- The dermis heavily guards the epidermis with
immune cells, e.g. macrophages, T lymphocytes etc
- The demis also consists of venules and lymphatic
vessels, providing entry to the blood circulation and drainage to regional lymph node

Question 7

What are the primary lymphoid organs? And elaborate on them.

Hint: Where the process happens, Shape,

Answer 7

Bone Marrow
- Site of haematopoesis, i.e. generation of blood cells
- In an embryo, this happens in amniotic sac
- In foetus, occurs in all bones, liver and spleen. Marrow is also very
cellular
- In adults, this occurs mostly in flat bones, vertebrae, Iliac bones, Ribs
and the ends of long limbs

Thymus
- Where maturity of T-cells occurs
- Bi- lobed
- Medulla and cortex regions
- No change during immune response to antigens, continuous development of T cells
- Hassalls’ corpuscle secretes soluble factors, and is important in regulatory T cells

Question 8

What is the problem of lymphocytes meeting antigen?

Answer 8

Large number of T cells with different specificities
Large number of B cells with different specificities
BUT limited amounts of antigen
How does the body ensure that the antigen meets lymphocyte with specific receptor?

Question 9

What is lymphocyte recirculation?

Answer 9

• Pathogen on mucosal surface
• Naive lymphocytes leave BM and Thymus and enter the bloodstream
• Recirculate through peripheral lymphoid tissue
• Recognition of antigen would result in massive B cell proliferation in secondary lymphoid tissue (lymphocyte activation)
• Otherwise the lymphocytes die

Question 10

Describe process of extravasion of naive T cells into lymph nodes

Answer 10

Extravasion of naive T cells into the lymph nodes (occurs during immune response)
- The naive T cell “rolls” along the
epithelium
- These are then stopped and
activated by specific chemokines at a particular place on the epithelium. This “right place” is determined by SELECTINS
-INTEGRINS then increase adhesion of the T cell to the epithelium, leading to arrest of the cell Transendothelial migration of the T cell from the bloodstream into the lymph node then occurs
Antigens also enter the lymph nodes via the draining lymphatics
Naive lymphocytes recirculate approx once per day – enter lymph node—high endothelial venue – lymphocyte is activated by antigen – stops recirculatng – massive proliferation of B lymphocytes – reenter the blood via the superior vena cava (via the efferent vessel) – target invading microbes/pathogen

Question 11

What are CD markers?

Answer 11

CD Markers
Found in T and B lymphocytes with agranular cytoplasm
• an internationally recognised systematic nomenclature for cell surface molecules
• used to discriminate between cells of the haematopoietic system
• more than 300 CD markers
• clinical importance e.g. CD4 in HIV

Question 12

Compare and Contrast phenotypic characteristics of B and T cells.

Answer 12

T Lymphocytes
• all express CD3- antigen specific receptor (TCR)
• gamma sigma TCR, about 10% in blood
• alpha sigma TCR, about 90% in blood: ~2/3 express CD4, ~1/3 express CD8. All mature T cells express one or the other
 CD4+ = T helper cells, regulatory T cells- Secrete cytokines
 CD8+ = cytotoxic T cells- Lyse infected cells, secrete cytokines
• Thymic output of naive T cells declines with age, and the thymus atrophies. Therefore older people have a
reduced ability to respond to new infections. However the total number of T cells does not change, there are just more memory cells.
ANTIGEN RECOGNITION
• only recognise processed antigen presented at the surface of another cell using T cell receptor
• antigen is presented by an MHC molecule

B lymphocytes
• Produced by and develop in bone marrow
• Surface antigen receptor (B cell receptor) : immunoglobulin like molecule
• Express CD markers CD19 & CD20 (not CD3, CD4 or CD8)
• Express MHC Class II (can present antigen to helper T cells)
• Effector function is to produce antibodies
ANTIGEN RECOGNITION
• recognise intact antigen free in body fluids (so not presented by another molecule)
• Use B cell receptor, a membrane anchored form of antibody linked to signalling subunits

Question 13

Give examples of antigen presenting cells (APCs) and their locations

Answer 13

Antigen presenting cells (APC)
cells that can present processed antigen (peptides) to T lymphocytes to initiate an acquired (adaptive) immune response:

Dendritic cells (DC)
- Location: Widely spread e.g. Skin & mucosal tissue
- Presents to T cells

B lymphocytes
- Location: lymphoid tissue
- Presents to T cells

Macrophages (activated)
- Location: lymphoid tissue
- Presents to T cells

Follicular dendritic cells
- Location: lymph node follicles
- Presents whole antigens to B cells