Lymphoid tissue Flashcards
What is a primary lymphoid organ?
Where lymphocytes are produced
What are examples of primary lymphoid organs?
Thymus, bone marrow, foetal liver
Where are B cells formed?
Bone marrow- primary site of haematopoiesis
Where does haematopoiesis occur in adults compared to children?
In foetus: occurs in all bones, liver and spleen
In adults: occurs in flat bones, vertebrae, iliac bones and end of long limb bones
Where are T cells formed?
Immature T cells formed in bone marrow and migrate to mature in thymus
Thymus derives thymocytes (T cells)
How is the thymus organised?
Highly organised with vasculature to allow molecules and cells to migrate in and out
What occurs in the thymus?
T cell selection process:
Positive selection- can TCR (T cell receptor) signal?
Negative selection- does cell react against our own body?
how does the thymus change with age?
Thymic involution: thymus shrinks/atrophies as we age
Functional tissue gets replaced with fat
Total thymic output of new T cells decreases but number of T cells in repertoire remains the same
What is a repetoire?
The range of genetically distinct RBCs or TCRs present in a given host. Larger the repertoire the more threats that can be recognised
What is a secondary lymphoid organ?
Where lymphocytes can interact with antigens and other lymphocytes
What are examples of secondary lymphoid organs?
spleen, lymph nodes, appendix, mucosal associated lymphoid tissue- all interconnected by lymphatic system and blood and are generally highly organised
What’s the role of a secondary lymphoid organ?
Brings cells closer in proximity to antigen
What is the role of the spleen?
Filter antigens that are found in blood
How is the spleen structured?
Has a white pulp = lymphoid tissue
Has distinct B and T cell zones
Has afferent and efferent lymphatic vessels
Has arterial and venous connections
What is the role of the lymph nodes?
Acts as a filter and slows down the flow of lymph
It contains lymphocytes which can trap and phagocytose any foreign antigen
How are the lymph nodes structured?
Lymph enters via afferent lymphatic vessels and leaves via efferent lymphatic vessels
B and T cells enter and leave via systemic circulation
Different regions for B and T cells
What is important about the medullary sinuses
A lymph node structure- contains macrophages
It provides a direct route to T or B cell areas depending on response needed
What is the role of mucosa associated lymphoid tissue?
Defends the epithelium
cutaneous immune system e.g. langerhan cells (DC)
What is a special structure of mucosa associated lymphoid tissue?
Peyer’s patches
These are specialised tissues in the gut (collection of lymphocytes- mainly B cells)
These are found below epithelium of ileum and small intestine
Follicle is highly enriched with B cells and has high frequency of germinal centres
What are germinal centres?
Anatomically restricted site where B cells undergo secretion to generate high affinity antibodies
What are dendritic cells?
Present antigen to T cells
Migrate to lymph nodes via afferent lymph vessels
How do T cells enter lymph nodes?
Through high endothelial venules- movement of T cells requires integrin and selected binding to high endothelial venules using L selectin on CD34
How are antigens transported from site on inflammation to lymph nodes?
- Uptake by professional antigen presenting cells- dendritic cells
- when DC uptakes antigen it migrates out of inflamed tissue and goes to lymph node carrying antigen towards B and T cell
- e.g. langerhan cell ad dermal dendritic cells can migrate in same way as DC via lymphatics and blood vessels - straight flow of antigen to lymphatics
