Organisation of immune system Flashcards
Barriers; Cell types; Lymphoid organs; Lymphocyte recirculation
What are the primary lymphoid organs and their functions?
Thymus and bone marrow
Where lymphocytes are produced by lymphopoiesis
What are the secondary lymphoid organs and their functions?
Spleen, Lymph nodes and mucosal associated tissues (MALT)
Where lymphocytes interact with antigens and each other
What is the structure of the thymus?
Two lobes → lobules containing Hassall’s corpuscles → fibroblast cells involved in the formation of regulatory T cells
What is a lymphoid organ?
An organ where lymphocytes develop and congregate
How does the thymus change with age?
Decrease in size
Reduced output
Less diverse T cells produced BUT total number produced same
Where is the thymus located?
Anterior-superior mediastinum
Behind sternum
Anterior to heart
Where does hematopoiesis occur in adults?
Bone marrow of Long/flat bones
e.g. Long bones of limbs, vertebrae, ribs
Where does hematopoiesis occur in the fetus?
All bones
Liver
Spleen
What are the two types of bone marrow?
Red - hematopoietic tissue
Yellow - Fatty tissue
How does hematopoiesis change during infection?
Increased white cell production
What are the functions of the lymphatic system?
Provides drainage to prevent build up of ECF
Involved in WBC interaction
What is the function of lymph nodes?
Filters the antigens in lymph
What is the general structure of a lymph node?
Lymphoid cells on outer edge containing B cells
(If ongoing immune response then B cells will proliferate and produce antibodies in the Germinal centres)
T cells closer to sinus than B cells, macrophages present to engulf pathogens
What changes in lymph nodes occur during infection?
Enlargement - due to proliferation of immune responses
What is the structure of the spleen?
White pulp surrounding blood vessels - contains lymphocytes
PALS - periarterial lymphatic sheath - surrounds arteries within white pulp, contains T cells
Primary follicles - adjacent to PALS - contain B cells
Red pulp - site of RBC turnover
What is the function of the spleen?
Filters antigens in the blood
What is the role of epithelia in the immune response?
1st line of defence - PHYSICAL barrier formed by mucosa and skin
What are the defence mechanisms associated with epithelia and why are they needed?
Lymphoid tissues associated with sites of likely infection
MALT/GALT/SALT
Needed because large surface area and may be single celled ∴ vulnerable
Describe the organisation of gut associated lymphoid tissue (GALT)
Special mucosal epithelium over Peyer’s patches (large aggregates of lymphocytes that drain to lymph nodes)
-Contains microfold cells to sample antigens in gut to present to lymphocytes for surveillance
Why are lymphocytes recirculated?
Lymphocytes with the correct specificity and antigen may not meet due to large number of T and B cells with different specificities
∴Cells travel through blood and lymphoid tissues until they meet antigen/die
What are the steps of lymphocyte recirculation?
1) Lymphocyte produced in 1° lymphoid organ
2) Lymphocyte travels to blood
3) Lymphocyte travels to tissue
4) Lymphocyte travels to 2° lymphoid organ
5) Back to blood (step 2)
What are high endothelial venules (HEV)?
Specialised vessels that allow entry of lymphocytes into the lymphatics
What is the process of HEV extravasation?
1) Rolling of naive T-cell along endothelium to HEV
2) Selectin (Tcell) binds weakly to endothelial CD34(HEV)
3) Chemokines on HEV bind to T-cell receptors ∴ integrins have higher affinity
4) Integrins bind and T cell enters tissue by transendothelial migration
What are the two major classes of lymphocytes?
B cells
T cells
How can you distinguish between lymphocyte classes?
System is used to discriminate hematopoietic cells based on their cell surface proteins
All B cells have MHC II, CD19 and CD20 (not 3,4,8)
All T cells have CD3 (some 4,8)
What form of antigens do B cells recognise?
Recognises intact antigens in body fluids or on cell surfaces using B cell receptors - membrane anchored form of antibody
What form of antigens do T cells recognise?
Processed antigens presented at surface of another cell using a TCR by a Major Histocompatability Complex (MHC)
List the cell types of the immune system
Neutrophils Eosinophils Basophils Monocytes/Macrophages Mast cells Dendritic cells NK cells B cells T cells
What are the functions and distinguishing features of neutrophils?
Multi-lobed nucleus
First cells to site of tissue damage
Phagocytosis
What are the functions and distinguishing features of eosinophils?
Phagocytosis Release granules Defend against parasitic infection Help with B cell responses producing IgA Stain with eosin
What are the functions and distinguishing features of basophils?
Release granules
May act as antigen presenting for type 2 immunity
Antibody mediated
What are the functions of monocytes/macrophages?
Important for phagocytosis
Release cytokines as mediators
Are antigen presenting for T cells
What are the functions of mast cells?
Release granules containing histamine
What are the functions of dendritic cells?
Capture and present antigens to T cells
What are the functions of NK cells?
Lyse infected cells
