Lecture 7 - Immune tissues Flashcards
What is haematopoeisis?
Haematopoiesis is the process by which all blood cells are generated
Very select group of cells in bone marrow can generate all the blood cells
Lymphoid and myeloid progenitors give rise to different lineages
This occurs in the bone marrow
What is the role of the thymus?
Two-lobed organ in upper mediastinum posterior to the sternum, medial to lungs and superior to the heart
Reaches its peak size by puberty. 20-30g in puberty and 5-7g by 75 years
Consists of a framework of epithelial cells that house lymphocytes
Immature T cells migrate from bone marrow to the thymus where they multiply and begin to mature
Only 2% of immature T cells that arrive in the thymus achieve the proper “education” to become mature T cells
The thymus houses lymphocytes and trains them to not recognise self antigens
What are secondary lymphoid tissues?
Immune responses occur in secondary lymphoid tissues (e.g. spleen, lymph nodes, Peyer’s patches)
Secondary lymphoid tissues are specialised sites for turning on the immune response
Lymph nodes
Spleen
Mucosal Associated Lymphoid Tissue (MALT)
Gut associated lymphoid tissue (GALT)
Nasal asociated lymphoid tissue (NALT)
What is the role of the spleen?
The spleen is the largest lymphoid organ
Directs immune responses to antigens in the blood
Important for clearance of red blood cells
Hyposplenism or asplenia not fatal, but predisposes to infection (especially of capsulated bacteria)
Splenomegaly common in chronic inflammatory diseases (malaria, Hodgkin’s disease etc)
Red pulp filters blood, removes old RBC and other cells from blood, contains macrophages and carries out extramedullary hematopoiesis during fetal development and in emergencies.
White pulp contains specific lymphoid microenvironments.
Describe the lymphatic system and its role
A drainage system involved in fluid balance, returning to the blood
excess interstitial fluid (approx. 3 litres/24h)
plasma proteins
Lymph capillaries
Originate as “closed tubes” in almost all tissues (except CNS, epidermis and cartilage)
Capillary wall constructed of overlapping endothelial cells that respond to fluid pressure
Lymphatic vessels (lymphatics) carry lymph and cells from peripheral tissues to draining lymph nodes (LN)
Lymph = plasma containing proteins and solutes that filters out of venules and capillaries due to hydrostatic/oncotic pressure
Tissue-derived pathogens might be carried in lymph
Cells carried in lymph include lymphocytes, DCs, macrophages. Some phagocytes may have engulfed pathogens in them.
Tumour cells use this pathway for spreading from its primary site metastasis
Increased blood flow and vessel permeability in response to inflammation or infection leads to increased lymph flow
Lymphatics
Form from joining lymph capillaries
superficial lymphatics follow superficial veins
flow into lymph nodes in axillary (armpit), inguinal (groin) or cervical (neck) areas where they drain into deep lymphatics
deep lymphatics follow main vessels
lymph nodes either side of the aorta (para-aortic) drain the paired organs, nodes lying anterior (pre-aortic) the gut etc
Describe lymph nodes
Pea-size immune structures, strategically placed around the body
A central component of host defense
Filter lymphatics
LN represent the anatomical meeting place for cells of immune system and their exposure to antigen
Follicles: B cell rich area
• Primary follicle: mature, naïve B cells
• Secondary follicle: “Germinal Center Reaction”
• Germinal centers (GC) develop in response to Ag stimulation
• GC are site of B cell proliferation, selection, affinity maturation and differentiation
Paracortex: T cell rich areas
Medulla: lymphatic sinuses, plasma cells, B –cells, macrophages
Describe cell movement to and out of tubes
Naïve lymphocytes enter via high endothelial venules and exit via efferent lymphatics
Antigen presenting cells enter via afferent lymphatics, scan for lymphocytes and exit via efferent lymphatics
Cell movement is caused due to membrane receptors pulling the cell towards areas rich in their ligands.
This is called chemotaxis.
Distinct structural (stroma) cells in the lymphnode produce distinct chemokines that attract either B or T cells – this is why the segregation occurs.
B cell areas are rich in CXCL13 which is the ligand of CXCR5, a receptor expressed on B cells.
T cell areas are rich in CCL21 and CCL19 which are the ligands of CCR7, a receptor expressed on naïve T cells but also dendritic cells (the cells that interact with T cells)
What are mucosa associated lymphoid tissues?
Large organised aggregates of lymphoid tissue in the digestive, respiratory, and genitourinary tracts.
Peyer’s patches
Organised aggregates of lymphoid cells in the ileum
Essential for monitoring bacteria populations and preventing the growth of pathogenic bacteria in the intestine
They contain a large population of antibody-producing plasma cells with numbers far exceeding those in the spleen, lymph nodes and bone marrow combined!
