Lymphoid Tissues

Question 1

Describe the 3 types of tissues surveyed

Answer 1

• circulating ‘sentinel’ immune cells: antigen presenting cells
• tissue resident cells: tissue macrophages, non-immune cells
• specialised lymphoid tissues: lymphatic organs, lymph nodes

Question 2

Describe lymphatic vessels

Answer 2

• contains lymph fluid
• connected throughout the body
• drains interstitial fluid and lymph from peripheral tissues back to circulation
• interlinked with circulatory system to allow for free movement from circulatory to lymphatic and vice versa nearer the heart

Question 3

Describe peripheral tissue drainage

Answer 3

• interstitial fluid and immune cells drain from tissues into lymphatic capillaries
• these intertwined with blood capillaries
• lymphatic capillaries -> lymphatic vessels -> lymph nodes -> circulation

Question 4

Describe the immune responses in lymphoid tissues

Answer 4

• sampling of peripheral tissues
• enables efficient specific immune responses via lymphocyte activation
• brings together sentinel immune cell, pathogen and adaptive immune cells

Question 5

Describe the primary lymphoid organs

Answer 5

• supports differentiation, proliferation and maturation of stem cells into immunocompetent cells
• location of immature lymphocyte development
• bone marrow, thymus

Question 6

Describe bone marrow

Answer 6

• origin of blood cells and location of initial B cell development - rest in spleen
• RBC, myeloid cells, lymphocytes
• positive and negative selection (BCR, self-antigen)

Question 7

Describe the thymus

Answer 7

• location of T cell development
• located in upper thoracic cavity - behind sternum, between lungs
• progenitor T cells from bone marrow migrate to thymus in early life
• thymocytes interact with epithelial cells within thymus to develop

Question 8

What are thymocytes?

Answer 8

T cell progenitors

Question 9

Describe T cell development

Answer 9

• thymocytes enter thymus via blood vessels
• undergo series of positive selection (cortex) and negative selection (medulla)
• selection involves MHC molecules and self-antigens

Question 10

Describe secondary lymphoid organs

Answer 10

• enable interaction between cells from circulatory systems and the lymphatic system
• primary site of specific immune responses
• spleen, lymph nodes, mucosa associated lymphoid tissues (MALT)

Question 11

Describe how secondary lymphoid organs enable interaction between cells from circulatory systems and the lymphatic system

Answer 11

• lymphocytes enter from blood via high endothelial venules (HEV)
• antigen presenting cells enter from lymph (afferent lymphatics)

Question 12

Describe the spleen

Answer 12

• functions as RBC filter, removing old cells - occurs in red pulp
• serves as site of B-cell maturation - occurs in white pulp
• white lymph nodes primarily filter lymph, spleen only filters blood
• contains antigen presenting cells in marginal zone of white pulp
• contains lymphocytes - resides in lymphoid sheath

Question 13

Describe the sections of the spleen where each cell is matured

Answer 13

• structure of white pulp encourages immune cell interactions
• central artery - blood access
• lymphoid sheath - t cells
• germinal centre - b cells
• marginal zone - dendritic cells and macrophages

Question 14

Describe the lymph nodes

Answer 14

• highly organised structure - similar to spleen
• filters lymph from peripheral tissues
• lymphocytes from blood can enter via HEV
• sites of specific immune responses
• formation of antigen specific T and B cells
• naïve lymphocytes circulate through nodes via chemokines CCL19 and CCL21
• B cells maintained in cortex via CXCL13 expression on stromal cells

Question 15

Which specific immune responses occur in the lymph nodes

Answer 15

• antigen presenting cells interact with T Cells
• specific T cells proliferate and interact with B cells
• specific B cells proliferate and generate plasma & form germinal centres

Question 16

Describe the different types of MALT

Answer 16

• MALT- lymphoid tissue associated with mucosa
• nasal mucosa - NALT
• gut mucosa - GALT
• bronchial mucosa - BALT
• skin - SALT

Question 17

Describe MALT

Answer 17

• protects mucosal surfaces of body - constant antigen exposure
• small, diffused lymphoid structures
• located throughout submucosal sites across body
• constitutes most extensive component of lymphoid tissue
• immunity dominated by IgA responses

Question 18

Describe MALT immunity dominated by IgA responses

Answer 18

• dimers protected by degradation and can cross epithelial barrier
• specialised lymphocyte recirculation and rehoming mechanisms
• e.g., CD103 binding to E-cadherin expressed by basolateral epithelial cells

Question 19

Describe GALT

Answer 19

• e.g., Peyer’s Patches
• line small intestine
• facilitates antigen sampling
• M cells - specialized epithelial cells and apical surface facing lumen
• similar function to lymph nodes
• prevents inappropriate responses
• locally produced cytokines key to maintaining immune regulation

Question 20

What do the dome structures in GALT contain?

Answer 20

• macrophages
• dendritic cells
• B cells
• T cells

Question 21

Describe how GALT has a similar function to lymph nodes

Answer 21

• activation of lymphocytes
• germinal centre formation

Question 22

Describe how how locally produced cytokines in GALT are key to maintaining immune regulation

Answer 22

• IL-10 and TGFβ anti-inflammatory
• secreted by T cells, dendritic cells
• TGFβ induces IgA production - can maintain food tolerance