Lymphoid Organs Flashcards
What are the primary lymphoid sites?
Bone marrow
Thymus
Foetal liver
What are the secondary lymphoid sites?
Spleen
Lymph nodes
Mucosa-associated lymphoid tissue (MALT)
What are some examples of mucosa-associated lymphoid tissue?
Tonsils
Gut-associated lymphoid tissue
Peyer’s patches in liver
Where is the thymus located?
Upper chest, behind sternum
What is the structure of the thymus?
It has two lobes
It is made up of lobules which are surrounded by connective tissue (capsules)
The lobules have an outer cortex and inner medulla
What is the difference between the cortex and medulla?
The cortex has a higher cell density, primarily full of immature T-cells
The medulla has a lower cell density, and contains more mature T-cells
What is the main responsibility of the thymus?
T cell development and selection
What is thymus central tolerance?
The process that trains T-cells to recognise self-antigens, to prevent autoimmunity.
90% of T cells are killed, and do not reach maturity
What are developing T cells known as?
Thymocytes
Where do thymocytes originate, and where do they migrate to?
Thymocytes originate in bone marrow, but migrate to the thymus for maturation.
How are maturing thymocytes regulated by the thymus?
Through positive and negative selection
Describe the positive selection in thymus
Thymoctyes must recognise and bind to MHC molecules- ensures T cells can interact with MHC molecules
Describe the negative selection in thymus
Thymocytes must not bind strongly to the MHC molecules that are presenting self-antigens- ensures no autoimmune reactions
What are MHC molecules?
MHC molecules are glycoproteins found on the surface of most cells in the body, acting as antigen-presenting molecules
What are the different outcomes of thymocytes?
- Most fail selection processes and undergo apoptosis
- Some that recognise self-antigens with moderate affinity are converted into regulatory T cells (natural Tregs)
-The remaining (that pass all selection tests) are released into the bloodstream as naive (mature) T cells
Are there regulatory mechanisms outside the thymus to ensure no autoimmune responses by T cells?
Yes. Tolerance (immune system’s ability to recognize and not attack the body’s own tissues and cells) is maintained outside the thymus by peripheral tolerance, e.g. by suppression by Tregs
What do mature T cells do after leaving the thymus?
Spend 1-2 mins in blood at a time, before entering secondary lymphoid tissues.
They bounce from one to another, until they find antigens of interest
What is the key roles of secondary lymphoid tissues?
- Limit pathogen spread (sieve like function)
- Permits efficient interactions between antigen, antigen-presenting cells, and immune cells
- Provides a controlled environment for the development of immune responses
What are lymph nodes?
Lymph nodes are small, bean-shaped structures that filter lymph and house lymphocytes.
Act as “security checkpoints” where immune cells check for pathogens
What is the function of the spleen?
The spleen filters blood, removes old or damaged blood cells, and helps fight pathogens in the blood.
What are B cell zones in secondary lymphoid tissue like lymph node?
Follicles where B cells proliferate
What are high endothelial venules (HEVs)?
HEVs are specialized blood vessels that allow lymphocytes to enter the lymph nodes
What are T cell areas in SLTs like lymph nodes?
T cell zones are areas where T cells interact with antigen-presenting cells
How are antigens delivered to lymph nodes?
Delivered via lymphatic vessels to lymph nodes
