4.2 Lymphoid Tissues Flashcards
What are primary lymphoid tissues?
The sites of lymphopoiesis
How is lymph formed?
Draining of interstitial fluid out capillaries and into lymphatic vessels
What are the 3 primary lymphatic organs?
Bone marrow
Thymus
Foetal liver
What cells are of the lymphoid lineage?
T cells, B cells and NK cells
Which part of the bone marrow does haematopoiesis occur on?
The red part
What does the yellow bone marrow do?
Contains mesenchymal stem cells that produce fat, cartilage and bone
Stores fat in adipocytes
Which cells mature in the bone marrow and which part of it?
B cells
Yellow marrow in the middle
Which cells mature in the thymus?
T cells
What is meant by thymic involution?
Shrinking of the thymus with age, leading to reduced T cell output
Functional tissue is replaced with fat
What are the three selection processes which the T cells undergo?
Positive, negative and final selection
What is positive selection?
Can the T cell receptor (CD4/CD8) signal
If not, the cell is destroyed
What is negative selection?
If T cell recognises self antigens, apoptosis is triggered
How do the T cells differentiate into T helper cells and T killer cells?
Depending on whether CD4 (helper) or CD8 (killer) has the strongest bond, the T cell will up regulate that one
How does the number of peripheral T cells change with age?
Remains the same
Peripheral T cells are maintained by division of mature T cells
Why are older people more vulnerable to new strains of pathogens?
Peripheral T cells are maintained by the division of mature T cells outside the central lymphoid organs
Results in less variety of T cells (fewer cells from new lineages)
How do T cells move from their site of production?
The thymus releases chemokines which initiates the chemotaxis of T cells from the bone marrow to the thymus
What happens to the thymus during infection?
No change
What happens to the bone marrow during infection?
Increased white cell production
What are secondary lymphoid tissues?
Sites where mature lymphocytes interact with antigens and other lymphocytes
What are some secondary lymphoid tissues?
Spleen
Lymph nodes
Appendix
Mucosal associated lymphoid tissue
What are lymph nodes?
Highly organised, encapsulated structures located at points of convergence of lymphatic vessels
What are germinal centers?
Anatomically restricted site where B cells undergo maturation and differentiation to generate high affinity antibodies
Through what vessel does lymph enter into the lymph nodes?
The afferent lymphatic vessel
Why do we have a lymphatic system?
More tissue is filtered than reabsorbed during tissue perfusion
Lymphatics return fluid to the systemic circulation
What are the functions of the spleen?
- Store of platelets
- Reacts with blood borne antigen
- Removes defective RBCs from circulation
What forms the first line of defense against infection?
Epithelial barriers
What is the significance of the location of lymphoid tissues?
They are spread around the body and interconnected via lymphatic system and blood
Thus they bring cells in close proximity to antigens
What are gut associated lymphoid tissue called?
Peyer’s patches – specialised secondary lymphoid tissues
Where are Peyer’s Patches Found?
Below the epithelium of the ileum of the small intestine
What are 2 features of Peyer’s patches?
Many B cell follicles with germinal centres
Areas between follicles occupied by T cells
Why are there a lot of germinal centers and secondary lymphoid tissues in the gut?
High amount of foreign antigens in the form of microbial products from the microbiota of the intestinal tract
What happens in the red pulp of the spleen?
Red blood cells are destroyed
What is a periarteriolar lymphoid sheath?
Area of white pulp surrounding arteries running through the spleen
Highly populated with T cells
Which special cells do gut associated lymphoid tissues contain?
M cells
What do M cells do?
Take up small amounts of gut antigen
Resident dendritic cells in Peyer’s patch present antigen to T cells
B cells react and produce IgA antibodies which are secreted into the gut lumen
What do the IgA antibodies made in the gut do?
They prevent microorganisms in the gut from sticking to the gut epithelium, and can also neutralize toxins and viruses
What forms the Waldeyer ring?
The pharyngeal, tubular, palatine and lingual tonsils
What are the tonsils made of?
Epithelial cells that phagocytose bacteria and transfer antigens to macrophages that act as APCs
How are tonsils involved in lymphatics?
Many lymphoid follicles with large germinal areas for B cell priming
Why is the waldeyer ring an ideal environment for the immune system to encounter antigens?
Food, bacteria and things we breathe in pass the oral pharyngeal section very often, therefore more antigens
Why do lymph nodes swell during an infection?
B cell follicles of the lymph nodes expand as the B cells proliferate to form germinal centers, amd the entire lymph node enlarges
Where do naive Lymphocytes go after they are produced?
They enter into the blood stream and migrate to the peripheral lymphoid tissues
How long does it take for a T cell to recirculate?
24 hours
How do T and B cells get inside the lymph nodes?
They enter via high endothelial venules
Where are high endothelial venules found?
In T cell zones of the lymph nodes
Which organ is mainly associated with filtering antigen out of blood?
Spleen
What are the four steps of T cells getting inside the lymph nodes?
- Rolling (along endothelial surface)
- Activation
- Adhesion
- Transendothelial migration
What molecule on T cells results in the rolling of the T cell before it enters the T cell zone of the lymph nodes?
L-selectin on T cell
What molecules binds to L selectin?
CD34
What chemokines are involved in activating LFA-1?
CCR7 on T cells recognises CCL21 on the endothelium of the high endothelial venule
What happens when LFA1 is activated?
The affinity for ICAM-1 and ICAM-2 is increased
What does ICAM allow?
The migration of the lymphocytes through the blood vessel wall
What are dendritic cells?
Potent antigen presenting cells that pick up and bring antigen from sites of infection to secondary lymphoid organs
How do dendritic cells help with the adaptive immune response?
They pick up antigens and present them to T cells in secondary lymphoid organs to activate the T cells
How do dendritic cells display the antigens?
Through MHCII molecules
What stimulates dendritic cells to engulf pathogens?
PAMPs at the site of infection stimulate the pattern recognition receptors on the dendritic cell
Thus the cell engulfs the pathogen and degrades it
What does receptor-independent macropinocytosis allow dendritic cells to do?
Take up extracellular material like virus particles and bacteria
Summarise the transport of lymphocytes and antigens in the lymphatics and blood stream
Free antigens and dendritic cells travel from the site of infection to the lymph nodes through the afferent lymphatic vessel
Activated lymphocytes proliferate and differentiate, becoming effector cells
Effector cells leave the lymph nodes via the efferent lymphatic vessel
They are carried by the blood to tissues where they will act
What is an example of a dendritic cell?
Langerhans cells in epidermis
Describe what is meant by ‘repertoire’ in terms of lymphocytes?
The range of genetically distinct BCRs or TRCs present in a given host
The larger the repertoire, the more threats can be recognised
What enables a variety of B cell receptors?
Immature B cells undergo VDJ recombination
Where in lymph nodes generally are the T cells found relative to B cells?
T cells - Towards inside (further from the marginal sinus)
B cells - Towards outside (closer to the marginal sinus)
What is it that makes the spleen so good at filtering antigens from the blood?
It is highly attached to the arterial circulation via the splenic artery
So it can filter blood rapidly and in large numbers
What does lymph do?
Transports dietary lipids, drains interstitial fluid and facilitates immune responses
What are the 2 steps involved in extravasation of naïve T cells into lymph nodes?
Selectin binding
Integrin binding
Define antigen presentation
The display of peptides in the major histocompatibility complex (MHC) I or II proteins such that the T cell receptor can attempt to bind them
How do dendritic cells transport antigens from site of inflammation to the secondary lymphocyte tissues?
Once dendritic cells uptake antigen
They migrate out of the inflamed tissue
Carry antigen towards the T cells and B cells
How do lymph nodes, the spleen and mucosal associated lymphoid tissues all function?
They trap antigens and APCs from sites of infection in order to present antigens to migratory lymphocytes
Thus they induce an adapative immune response
