Organs of the Immune System

Question 1

Primary lymphoid organs

Answer 1

• Extremely important for adaptive immunity
• Thymus
• Bone marrow
• Bursa fabricius in birds
• Ileal Peyer’s patch
• Lymphoglandular complexes

Question 2

Function of primary lymphoid organs

Answer 2

Sites where lymphocytes develop

Question 3

Thymus location

Answer 3

Located in the thoracic cavity, near the heart and can extend into the neck region

Largest in the newborn, slowly decreases towards adulthood

Question 4

Thymus structure

Answer 4

• Consists of lobules of loosely packed epithelial cells.
  o Each lobule is covered by connective tissue capsule
  o Cortex: outer part of lobule. Densely packed with lymphocytes and epithelial cells
  o Inner medulla: fewer lymphocytes and fewer epithelial cells
  o Hassall’s corpuscle: structure in the medulla containing blood vessel but walls very thick preventing antigen to enter the thymus
• No lymphatics leaving the thymus

Question 5

Function of the thymus

Answer 5

• Source of the thymus-derived lymphocytes (T cells)
• If removed, animals would die because of lack of adaptive immunity

Question 6

T-cell Selection that occurs in the thymus

Answer 6

• Positive- recognize own MHC
• Negative- don’t want them to recognize own peptides
• **Remember: only about 2-4% of T-cells in mouse study actually left the thymus

Question 7

Bursa fabricius

Answer 7

• Only found in birds
• Round sac above the cloaca
• Largest in young birds and then decreases in size
• Lymphocytes embedded in epithelial tissue and the epithelial tissues line a hollow sac connected via a duct to the cloaca
  o Sac full of lymphocytes extends into the lumen = follicle
  »Follicles divided into medulla and cortex

Question 8

Function of bursa fabricius

Answer 8

• Source for B cells in birds
• Bursa also traps foreign antigen which is not normal for primary lymphoid organs

Question 9

Ileal Peyer’s patches (types)

Answer 9

1. Jejunum
2. Ileal

Question 10

Peyer’s patches location

Answer 10

Located in the wall of the intestine

Question 11

Jejunum patches

Answer 11

• Inductive site and antigen uptake
  o Filtration of lymph, antigen presentation, activation of naïve lymphocytes, clonal expansion
• Multiple distinct patches
• Develop before birth

Question 12

Ileal patches

Answer 12

• Site of lymphocyte development (primary lymphoid organs)
• One continuous patch
• Largest shortly before birth, place where B cell development occurs in mammals

Question 13

Peyer’s patches development in other species like primates, rabbits, rodents

Answer 13

Develop after birth in response to microbiota

Question 14

Secondary lymphoid organs

Answer 14

• Lymph node
• Spleen
• Tonsil
• Jejunal Peyer’s patches

Question 15

Function of secondary lymphoid organs

Answer 15

• Filter lymph
• Site of antigen-presentation
• Activation of lymphocytes
• Differentiation and maturation of antibody specific cells

Question 16

Lymph nodes

Answer 16

• Filters for free antigen and sites for antigen presentation to naïve B-cells and T-cells
• Lymph flows in through afferent vessels, and out through efferent vessels
• Leukocytes enter through high endothelial venuoles (HEV) or lymph

Question 17

Lymph node structure

Answer 17

Lobules
- Follicles with germinal centers
- Paracortex (Interfollicular region filled with T cells)

Afferent vessels (entrance)
- Entrance into the lymph node. Antigens are coming in and there are B-cells and antigen-presenting cells there to grab them.

Efferent vessels (exit)

Question 18

Residential cells of the lymph nodes

Answer 18

• Dendritic cells (follicular)
• Follicular T helper cells

Question 19

Where are the T cells and B cells in the lymph nodes?

Answer 19

• T cells in the paracortex
• B cells in the cortex

Question 20

Germinal centers

Answer 20

Areas of the lymph nodes where B cells start to mature and somatic hypermutation occurs

Question 21

Germinal centers size

Answer 21

Larger when they are more active. Occurs because full of B cells undergoing somatic hypermutation. Results in swollen lymph nodes.

Question 22

Steps of somatic hypermutation

Answer 22

1. Activated Th cells migrate to follicles, differentiating into follicular Th cells
2. B cells are activated, some enter the germinal centres to undergo isotype switching and affinity maturation. At the same time, some will leave and go make antibodies
3. B cell somatic hypermutation occurs in the dark zone. Antibody affinity tested in light zone
4. Follicular Th cells will tell the B cells to undergo further affinity maturation OR to undergo apoptosis if they are not positively selected for
5. B cells clones with highest antigen affinity differentiate into plasma cells and memory cells

Question 23

Common structure of tonsils and Peyer’s patches

Answer 23

• Dome region: macrophages and dendritic cells
• Follicles: B cells and follicular dendritic cells. Edge contains Th cells
• Interfollicular region: T cells

Question 24

General functionality of tonsils and Peyer’s patches

Answer 24

• Antigen gets trapped or taken up by M cells
• Antigen presenting cells present the antigen to the T cells (through 3 signals) in interfollicular region
• Follicular dendritic cells “hold it” for B cells. Follicular cells form a network of dendritic cells, and when the antigen gets filtered through they grab it and hold it out for B cells to recognize and bind to it, if their B cell receptor is specific for the antigen. (NOT PRESENTED BY MHC)

Question 25

Cell layer and location of tonsils and Peyer’s patches

Answer 25

• Found at mucosal sites
• Covered by one layer of epithelial cells

Question 26

Function of Spleen

Answer 26

Filters the blood and stores RBCs and platelets

Question 27

Structure of the spleen

Answer 27

Has red pulp and white pulp separated by marginal zone
- Marginal zone is rich with antigen presenting cells
- White pulp is area full of T cells which surround the incoming arterioles
- B cells are in follicles scattered around lymphoid tissues

Question 28

Red pulp vs. white pulp

Answer 28

• Red pulp: filtering and storage of red blood cells
• White pulp: White blood cells (T and B cell rich)