Organisation of the Immune System Flashcards
Barriers: recall the physical and chemical barriers used by the immune system in host defense Cell types: list the different cell types of the immune system, explain how they may be distinguished, and briefly summarise their functions Lymphoid organs: recall the primary and secondary lymphoid organs, summarise their structure, and differentiate between their functions Lymphocyte recirculation: summarise the re-circulation of lymphocytes, and explain its importance
What are primary lymphoid organs?
Where lymphocytes are produced by lymphopoiesis (lymphoid stem cells differentiate to mature functional lymphocytes).
List examples of primary lymphoid organs. (x2)
Thymus and bone marrow.
What are secondary lymphoid organs?
Where lymphocytes interact with antigens and each other to generate an immune response.
List examples of secondary lymphoid organs. (x3)
Spleen, lymph nodes and mucosal associated lymphoid tissues (MALT).
What are alternative names for primary and secondary lymphoid organs?
PRIMARY: generative lymphoid organs. SECONDARY: peripheral lymphoid organs.
What are ALL lymphocytes derived from?
From precursor bone marrow stem cells. By the way, all the next cards about lymphocytes relates to the ADAPTIVE IMMUNE system.
What is the anatomical position of the thymus?
Found below the thyroid in the thoracic cavity.
What is the structure of the thymus?
Has two lobes. Septum divides lobes into lobules. Dark-staining regions around the outside of the lobule = cortex. Light-staining region in the middle = medulla (medulla means middle).
What is the function of the Hassall’s corpuscles?
Have fibroblast cells, so involved in the formation of regulatory T-cells.
What is the function of the thymus?
Performs an immune function. Packed with MATURING and PROLIFERATING T-lymphocytes.
What is the function of the bone marrow?
Site of haematopoiesis – the process of differentiation and maturation of ALL blood cells.
What happens visibly to the thymus and bone marrow during infection?
Thymus – no obvious change during infection. Bone marrow – visible change during infection where there is increased white cell production.
What is the difference between a foetus and adult in relation to sites of haematopoiesis?
Foetus: bone marrow very cellular, and liver and spleen involved. Adults: large, flat bones are involved. Not the liver or spleen.
How many litres are returned to the blood from the lymphatic system each day?
2-3 litres per day.
What is the function of lymph nodes – how is this exercised?
Nodes filter antigens in the lymph – see if immune system recognises any of those antigens. Afferent lymphatic vessels bring lymph fluid into the node, while efferent leaves the node. Fluid collected is filtered through lymphocytes so can react if antigens are present.
What is the structure of the lymph node?
Refer to photo. Artery and vein lead in and out. Note the afferent and efferent lymphatic vessels.
Where are immune cells found in the lymph node? (x3) What does enlargement of the lymph node denote?
Lymphoid cells on the OUTSIDE tend to be B cell aggregates in the follicles. If ONGOING immune response, B cells will proliferate and produce antibodies in germinal centres [yellow circles]. Germinal centres are distinct spheres of proliferating B-cells. T-cells are closer to the medullary sinus than the B cells, and macrophages are present to engulf pathogens. Enlargement demonstrates ONGOING immune response and hence proliferation of immune responses.
What do germinal centres indicate?
Germinal centres show ongoing immune response. [This is basically emphasising the previous flashcard.]
What is a high endothelial venule? Organ where there are no HEVs?
Specialised area of ARTERY with much thicker endothelium that causes T/B-cells to leave circulation and ENTER the lymph node following chemical signals from chemokines. SPLEEN does NOT have high endothelial venules.
What is the function of the spleen?
Filters antigens in the blood (while lymph nodes do the same but for LYMPH).
What is the anatomical position of the spleen?
Look at photo.
What is the structure of the spleen? Histologically?
White pulp is where lymphocytes reside, and red pulp is where erythrocytes present. Periarterial lymphatic sheath (PALS): immediately surrounds the arteries (in white pulp), with predominantly T-cells. Primary follicles possess primarily B-cells. Phagocytes also present and scavenge for dead cells/pathogens. Histologically, white pulp stains dark purple, and red stains lighter.
What are mucosal associated lymphoid tissues?
These describe the PHYSICAL BARRIES that prevent against invasion from pathogens in the first place. Most of MALT has a high surface area. Because they are the physical barriers, they are heavily defended by the immune system. Include the epithelium of the skin and the surface of the gut.
Describe the lymphatic system in relation to the gut.
Epithelium of the gut is an example of MALT.
□ Intraepithelial lymphocytes reisde on villus. These villi have lymphatic drains to mesenteric lymph nodes.
□ Peyer’s patches (found only in the gut) are large aggregates of lymphocytes and follicles especially B-cells, that drain to lymph nodes.
□ Specialised mucosal epithelium over peyer’s pathces contain microfold cells which sample antigens in gut and present to lymphocytes in peyer’s patches.
What is the process of extravasation of naïve T-cells into lymph nodes?
- Naïve T-cells roll along the endothelium until they reach the HEV (high endothelial venule). 2. Selectin (on the T cell) binds weakly to endothelial CD34 (on the lymph node endothelium in the HEV). This allows rolling interaction. 3. Chemokines present on surface of HEV changes the conformation of integrin on T-cells, which means that the T-cell has a high affinity binding to the endothelium. 4. T-cells move over endothelium into lymphoid tissue. Called transendothelial migration.
What do lymphocytes look like? (x2 points about histological appearance)
Small cells with agranular (lacking granules) cytoplasm and large nucleus.
What CD (cluster of differentiation) markers do T-lymphocytes express?
They ALL express CD3. There are two main types of T-lymphocytes: those with an alpha-beta TCR (T-cell receptor) (90% of all T-cells), and those with a gamma-delta TCR (10% of T-lymphocytes). abTCR: 2/3 have CD4; 1/3 have CD8. CD4 and CD8 are CO-RECEPTORS.
Don’t worry about the others.
What do neutrophils look like?
Phagocytes with multi-lobed nucleus (polymorphonuclear granulocytes).
What is the mechanism by which Natural Killer cells lyse other cells? What two ways can a virus affect a cell in relation to this mechanism?
Have activating and inhibitory receptors which BALANCE signals and dictate whether target cell is lysed or left alone.
Normal, unaffected cell contains MHC Class I molecules (they are a marker of normal self). However, in a virus infected cell, the virus can downregulate these molecules, so cell no longer gives an inhibitory signal to the NK cell. Stress-induced molecules are also expressed on the cell. So, there is a loss of inhibition, and activation receptors are stimulated – so NK cell lyses the cell as it detects that the cell is ‘missing self’ and there’s a problem.
