Anat - Immune System

Question 1

What is the lymphoid/lymphatic system?

Answer 1

Consists of cells, tissues, and organs involved in the defense of the body against invasion by bacteria, viruses, and other foreign bodies

Question 2

What is a lymphoid tissue?

Answer 2

Reticular connective tissue filled with numerous lymphocytes

Can be classifed as primary vs secondary, and diffuse vs dense

Question 3

What are examples of primary lymphoid tissues?

Answer 3

• Bone marrow (naive B cells develop here)
• Thymus (T cells develop here)

*These are where lymphoid cells are generated/developed

Question 4

What are examples of secondary lymphoid tissues?

Answer 4

• Lymph nodes
• Spleen
• Tonsils
• Lymphoid aggregates in certain organs
• Scattered lymphocytes in connective tissue
• Lymphocytes circulating in blood and lymph
• Other immune cells: plasma cells, APCs (macrophages, langerhan’s cells of the skin, DC in lymph nodes/spleen, microglial cells in CNS)

*These are where lymphoid cells reside and function to fight antigens

Question 5

What are examples of diffuse and dense lymphoid tissue?

Answer 5

Diffuse - lamina propria of gut and respiratory tract

Dense - Mucosa associated lymphoid tisses (MALT), discrete encapsulated organs
=> Dense lymphoid tissues are capsules of lymphoid cells

Question 6

Compare and contrast innate and adaptive immunity

Answer 6

Innate:
- Non-specific
- Granulocytes/leukocytes
- Early response

Adaptive:
- Specific
- T and B lymphocytes
- APC (from monocytes)
- Memory lymphocytes pdn
- More gradual, slow response
- Cellular or humoral (antibodies) types

Question 7

What are the physicial barriers of the innate immunity?

Answer 7

1. Intact skin
2. Mucous membrane
   - Luminal surface of cavities are lined by mucous membrane (mouth, nose, vaginal, anal)

Question 8

What are the chemical barriers of the innate immunity?

Answer 8

• Hydrochloric acid of the stomach
• Lysozyme in bodily secretions such as sweat, tears, saliva
• Lactic acid in the vagina

Question 9

Explain how inflammation (non-specific) may occur?

Answer 9

Mast cells release histamine, cause vasodilation and increased permeability of the blood vessels, chemotaxis of WBCs to where pathogens are, phagocytes can consume pathogen and cell debris

Question 10

Describe the appearance of mast cells

Answer 10

Circular cell with circular nucleus
Granules filled with histamine + heparin

Question 11

Describe the appearance of macrophages

Answer 11

Contain lysosomes* and myelin figures

Question 12

Where does the adaptive immune response occur?

Answer 12

Occurs in specialized tissues and organs such as the lymph nodes and spleen

Question 13

Explain the movement of fluid across the lymphatic system

Answer 13

1. Part of the fluid from circulating blood in the capillaries passes into the surrounding tissues as tissue fluid
2. Some fluid can re-enter the capillaries in the circulatory system, while the rest enter the lymphatic vessels
3. Fluid in lymphatic vessels is known as lymph
4. Lymph nodes (bean-shaped) are scattered along the lymphatic vessels
5. Lymph from any part of the body must pass through one or more lymph node before it can enter the bloodstream

Question 14

What are the main lymphatic vessels

Answer 14

Main vessel: thoracic duct

Join back to the circulatory system through the subclavian duct into the subclavian trunk => to the smaller lymphatic vessels

Question 15

Compare the appearance of a resting lymphocyte to an activated plasma cell

Answer 15

Resting lymphocyte:
- Organelles are spread around the cell body
- Little cytoplasm, scanty organelles, scattered free ribosomes
- Nucleus contains predominantly heterochromatin clumps

Plasma cell:
- Organelles are concentrated in one part to make antibodies
- Abundance of protein-synthesizing rough endoplasmic reticulum

Question 16

B lymphocytes constitute about ____% of the circulating lymphocytes, while T lymphocytes constitutes ____%

Answer 16

B lymphocytes: 20-30%
T lymphocytes: 60-80%

*NK cells is a large granular lymphocyte (also come from the lymphoid steam cell)

Question 17

What are the 3 types of T lymphocytes?

Answer 17

1. Cytotoxic T cells (CD8+) - secrete perforin that forms a pore in the target cell membrane
2. Helper T cells (CD4+) - help to activated B lymphocytes
3. Suppressor T cells - switch off the immune response once infection is neutralized

Question 18

Explain the differentiation and development of B cells

Answer 18

Naive B cells develop in the bone marrow
Naive B cells express B cells receptors/surface Ig, leave bone marrow and enter circulation + secondary peripheral lymphoid tissues

Question 19

Explain the differentiation and development of T cells

Answer 19

T lymphocytes progenitors/precursors travel from the bone marrow to the thymus
They development into T lymphocytes in the thymus, before being localized in secondary peripheral lymphoid tissues

Question 20

[THYMUS]

Thymus is a primary lymphoid organ

Where is the thymus located?

Answer 20

Located above and anterior to the heart, between the lungs
In the anterior, superior mediastinum

Question 21

[THYMUS]

Compare b/w the appearance of an older and younger thymus

Answer 21

Older: fibrofatty infiltration (more fatty and fibrous)

Why?
Older - less activated, since have seen many antigens
Younger - more active when encounter new antigens

Question 22

[THYMUS]

Describe the structure of the thymus

Answer 22

1. Covered by connective tissue capsule
2. Has two lobes further divided into small lobules
3. Each lobule has an outer region called cortex and an inner part called medulla
4. No afferent lymphatic vessels, only efferent vessels carry lymph from thymus out into lymph node => circulation

Question 23

[THYMUS]

What are the cell types present in the thymus?

Answer 23

• Lymphocytes
• Epithelial reticular cells (EC) aka thymic epithelial cells
• Macrophages
• Endothelial cells

Question 24

[THYMUS]

What are the structures in one lobule of the thymus

Answer 24

*Outer to inner:
1. Capsule (connective tissue)
2. Cortex (outer)
3. Medulla (inner)
4. Trabeculae (innervations)

Question 25

[THYMUS] Describe the appearance, contents, and roles of the thymus cortex

Answer 25

Appearance: - Peripheral dark staining region of each lobule (compared to lighter medulla) Contents: - Main cell type: small lymphocytes - EC less numerous than in medulla - EC ensheath the cortical capillaries (*the only blood vessels in the cortex) Role: - Site of T lymphocyte precursor proliferation and the blood-thymus barrier

Question 26

[THYMUS] Describe the appearance, contents, and roles of the thymus medulla

Answer 26

Appearance: - Lighter central region Contents: - More EC - Less lymphocytes - Hassall's corpuscles (cluster/grp of ECs) Role: - Mature lymphocytes enter circulation in the medulla or the cortico-medullary junction to populate T-dependent areas of other lymphoid organs - EC produce thymosin and thymopoetin which promote T cell differentiation and proliferation

Question 27

[THYMUS] What is the function of the thymus?

Answer 27

1. T lymphocyte differentiation 2. Selective removal of T cells reactive against self-antigens - Positive selection in the cortex: survival of T cells with function TCR recognizing both MHC I (CD8) and II (CD4) molecules - Negative selection in the medulla: survival of T cells that do not tightly bind self-antigens presented by dendritic cells => Degeneration of self-reactive T lymphocytes (80%) + maturation of self-tolerant T-lymphocytes (20%) 3. EC secretes hormones like thymosin and thymopoetin which promote T cell differentiation and proliferation

Question 28

[LYMPH NODES] Describe the structure of a lymph node

Answer 28

Structure: - Bean shaped structure, convex and concave surfaces - Hilum at concave surface (efferent vessels at hilum) - Has afferent and efferent vessels Outer to inner: - Afferent lymphatic with valve - Capsule and trabeculae (septa) - innervate transversely into lymph nodes - Superficial Cortex - Deep cortex - Paracortical zone - b/w cortical nodules & medulla (consist mainly T lymphocytes) - Medulla - Medllulary sinuses, drain into efferent lymphatic vessels

Question 29

[LYMPH NODES] What are the sinuses present in the lymph node?

Answer 29

Outer to inner: 1. Subcupsular/Marginal (afferent drains into here) 2. Trabecular/Cortical 3. Medullary (drains into efferent) *Sinuses are lighter in colour

Question 30

[LYMPH NODES] Describe the content, and role of the cortex

Answer 30

Content: - Mainly B lymphocytes arranged in lymphatic follicles (nodules) with germinal centers - Other cell types: plasma cells, macrophages, antigen presenting DCs, reticular cells (RCs are a type of fibroblast for structural support) Role: - Germinal centers: lighter zone in which secreting plasma cells and memory B cells are produced

Question 31

[LYMPH NODES] Describe the content, and role of the medulla

Answer 31

Content: - Consists of medullary cords and sinuses => Medullary cords are continuous and consist of: lymphocytes (mostly B cells), plasma cells, reticular cells, dendritic cells => Medullary sinuses are vessel-like spaces that separate the cords, they receive lymph from subcapsular and trabecular sinuses, and drain into the efferent lymphatic vessels at the hilum

Question 32

[LYMPH NODES] What is the function of the lymph nodes? List some examples of where lymph nodes are located?

Answer 32

1. Lymph circulates through the lymphatic vessels and the lymph nodes 2. Foreign substances in the lymph are trapped by APCs in the lymph nodes Located: neck, axilla (armpit), inguinal (groin) region

Question 33

[LYMPH NODES] Describe the circulation of lymphocytes (in lymphatic fluid and in blood) *How do they enter and leave the lymph node?

Answer 33

1. Lymphocytes circulate through the lymphatic AND blood vessels 2. Lymphocytes in the LYMPHATIC FLUID enter lymph node through afferent lymphatic vessels 3. Lymphocytes in the BLOOD enter through the walls of postcapillary venules (aka high endothelial venules) in the lymph node 4. Lymphocytes leave the lymph node via efferent lymphatic vessels *venules are located at the paracortex

Question 34

[SPLEEN] What is the spleen? Where is it located?

Answer 34

Spleen is the largest single lymphoid organ that serves as an immunologic filter for blood. It is the site of destruction of old RBCs. *Humans can survive w/o the spleen Spleen is located at the upper left of the abdominal cavity

Question 35

[SPLEEN] What are the structures in the spleen?

Answer 35

- Capsule - Lymph vessels - Central artery - Trabecular artery and vein (innervations) - Lymphatic/Splenic nodule (white pulp) - Splenic cord (red pulp) - Venous sinus (red pulp)

Question 36

[SPLEEN] Describe the structure, content and role of the white pulp

Answer 36

White Pulp - Splenic Nodule (25%) *lighter stain Structure: - Consists of lymphoid tissue in the form of lymphoid follicles and periarterial lymphatic sheaths (PALS) surrounding the central arteries Content: - PALS contain mainly T lymphocytes - Lymphoid follicles contain mainly B lymphocytes and develop germinal centers Role: - Produce WBC

Question 37

What is the difference between the lymphatic nodule of the lymph node and the lymphatic nodule of the spleen?

Answer 37

Lymphatic nodule of the spleen is perforated by an arteriole (central artery) Though both consist mainly B lymphocytes, and have germinal centers

Question 38

[SPLEEN] Describe the structure, content and role of the red pulp

Answer 38

Red Pulp - Splenic cords (76-79%) *darker stain 1. Red pulp cords (Billroth's cords) - B/w the sinusoids, serve as reservoir of blood cells, macrophages, lymphocytes, plasma cells, reticular fibers 2. Splenic sinusoids - Wide and irregular lumen - Discontinuous lining of endothelial cells (stave cells) - line the venous sinuses to filter blood Role: - Filter blood (remove microorganism, debris, worn out RBCs)

Question 39

What is the medical important of the immune system?

Answer 39

- Transplantation - Immunopharmacology - Immunization/vaccination - Allergic reactions

Question 40

What are some other examples of lymphatic tissues in other organs?

Answer 40

1. Peyers patches - Lymphatic nodule in the lamina propria of ileum, that may extend into the submucosa - Protects SI from infection (since ileum directly connected to cecum where a lot of bacteria reside) 2. Palatine Tonsil - Lymphatic tissue in the oropharynx that prevents infection in the respiratory and digestive tracts - Consists of lymphatic nodules - Stratified squamous epithelium - multiple layers to protect against constant abrasion