L21. Lymphoid System Flashcards
What is the thymus
What are the components in the thymus
Connective tissue forms the outer ‘capsule’ which is mesodermally derived.
Reticular epithelial portion (endodermally-derived)
-reticular because the epithelium is not continuous
- forms ‘net-like’ structures throughout the thymus
Describe each lobule in the thymus
- outer basophillic cortex : cortex contains T-cells
- inner medulla (eosinophillic-pink) has HC (Hassall’s Corpuscles
What are Hassall Corpuscles , where are they found
- found in medulla of a thymus lobule
- HC clusters of reticular epithelial cells in medulla
- produce cytokines for T-cell development
What is the cytokine for T-cell developmengt
thymopoietin
What occurs at the thymic lobule?
-T-cell development
L21 pg 7a identify the medulla, epithelial reticular cells , Hassall’s corpuscle , cortex and lymphocytes in the thymic lobule
pg 7a L 21
Describe the post-capillary venules at the thymic lobule
- located at the corticomedullary junction
- no blood-thymus barrier
- T lymphocutes from marrow enter thymus
- exit of naive immunocompetent T-cells to secondary lymphoid organs
Describe the epithelial reticular cells in the thymic lobule
- contribute to the blood-thymus barrier around continuous capillaries in cortex to immunoprotect developing T-cells
- work with dendritic cells to present antigens that regulate clonal deletion of pre-T cells at cortico-medullary junction
- become conentrically arranged as Hassall’s corpuscles in medula; produce thymopoietin
Name a secondary lymphoid organ
lymph nodes
Describe the lymph nodes
- surrounded by a dense irregular CT capsule and trabeculae
- true reticular CT (collagen 3) stroma
- have afferent and efferent lymphatics
- in the path of ‘lymph vascular flow’
What are primary lymphoid organs
- sites of lymphocyte production
- naive, immunocompetent B lymphocyte produced in bone marrow
- naive immunocompetent T lymphocytes are produced in bone marrow and thymus
What are secondary lymphoid organ
-lymphocite activation occurs (foreign antigens are encountered/presented)
Name secondary lymphoid organ
- Lymph node
- mucosa-associated lymphoid aggregrates (M.A.L.T)
- spleen
What is the MALT? Describe it
- Mucosa-Associated Lymphoid Aggregates.
- sub-epithelial lymphoid aggregates
- no CT capsule
- not in lymph vascular flow
Describe the spleen
- dual function; lymphocyte activation and aged RBC destruction
- surrounded by CT capsule,not in lymph vascular flow
What is the difference between B-lymphocytes and T-lymphocytes
- B lymphocytes develop in the bone marrow. T lymphocytes develop in the bone marrow and thymus
- Lymphocytes proliferate and become lineage restricted to generate many different clones of
- B lymphoblast that remain in the bone-marrow
- T lymphoblast that migrate to the thymus - B lymphocyte rearranges Lg genes and inserts proteins into PM ->naive pre-B-cells
T lymphocyte rearranges T-cell receptor genes into PM where they can recognize a single, specific antigen presented to them by APC - The Ig of the pre-B-cell interacts with self-antigens in the bone-marrow this induces apoptosis. (ensure there is a tolerance to self-antigens). If the TcR of a naive pre-T-cell reacts with self-antigens presented by APC in the thymus this induces apoptosis, this helps ensure there is a tolerance to self-antigen
- remaining naive, immunocompetent B-cells leave bone marrow via venous sinusoids. they are ready to be activated by foreign antigens to produce soluble Igs (antibodies).
immunocompetent T-cells leave thymus and enter post-capillary venules at the corticomedullary junction and are activated by APCs in secondary lymphoid organs
What are dendritic cells?
- derived from monocytes/macrophages in CT
- are APC
- bridge between innate and adaptive immune response
- digest non-self proteins and carbohydrates
- present them as very specific antigens on their surface to lymphocytes
What is diapedesis
extravasation ( leakage of fluid). how WBC move from vessels into CT
Describe the process of diapedesis
capture ( through tethering)-> rolling ->slow rolling-> firm adhesion ( by integrins) ->transmigration
- tethering of WBC and rolling –selectins
- strong adhesion and migration across endothelium– integrins
How is WBC recruitment mediated
- cytokines : paracrine factors released by macrophages and neutrophiles, NK cells
- Dendritic/ Monocytic cells (APC)antigen presenting cells. present antigens from pathogens to lymphocytes that exit the blood by diapedesis
What are the cytokines released by macrophages and neutrophils
interleukins
What are the cytokines produced by Natural killer cells
interferons
What are complement proteins
- plasma proteins made in the liver
- released into CT due to blood vessel injury
- binds to lipids/ carbs on pathogens
- binds to receptors on macrophages /neutrophils
What are Toll like receptors
- on surface of macrophages and neutrophils
- recognize various pathogen molecules non-specifically (LPS, glycoproteins, dsRNA)
What are some non-specific pathogen molecules
- LPS
- glycoproteins
- dsRNA
Who initiates innate immunity
- phagocytic activity of macrophages and neutrophils
- Natural Killer Cells facilitate cell-mediated killing of target cells
Describe the Natural killer cells
- recognize and bind to target cells ( virus-infected cells ) with low specificity
- transfer pore-forming ‘perforins’ to bound target cell membranes; kills target cell
- releases interferon gamma, a cytokine that stimulates macrophages
What are the 2 types of immunity
- innate immunity
2. adaptive immunity
Describe innate immunity
- natural immune system ; rapid response
- non-specifically eliminates pathogens
- initiated by macrophages and neutrophils, facilitated by granulocytes
Describe adaptive immunity
- ‘acquired’ immune system; slower response
- responds to highly specific molecular epitopes = antigens
- mediated by B and T lymphocytes; develop in primary and secondary lymphoid organs
Describe the lymph interstital fluid
- plasma like
- foreign antigens
- lymphocytes
- antigen-presenting cells
Describe the lymph vascular system
- moves interstitial fluid (lymph) from connective tissues back to the venous system
- empties into subclavian veins in the root of the neck
- begins at lymphatic capillaries
- blind-ended, one way flow
- NO TIGHT JUNCTIONS ( intercellular clefts between endothelial cells facilitate movement in/out no tight junctions
Describe the tissue organization of the lymph vessels ( TI,TM,TA)
- TI fold to form one way valves in the direction of flow back to the venous system
- thin walled for all 3 organization: TI, TM, TA
- neuro vascular bundle in dense irregular connective tissue
Describe lymph flow in a lymph node
lymph moves through sinuses ( spaces from outer cortex to inner medulla)
What occurs at the paracortex of the lymph nodes
activation of T cells to form CD-8 Cytotoxic T cells and Helper (CD-4) T cells (help activate B cells)
What happens at the cortex/nodule in a lymph node
The activation of B cells to either plasma cells ( secrete antibodies or memory cells ( rapid secondary response)
What are CD-8 T cells
Cytotoxic T cells
What are CD-4 T cells ?
Helper T cells
How do T and B cells enter and exit the thymus?
post capillary venules in the paracortex
Identify the cortex/nodules, subcapsular sinus, paracortex,medulla, medullary sinus, lymph (in out) and post capillary venules on PAGE 9 of LECTURE 21
page 9 lecture 21
Name examples of Mucosa Associated Lymhoid Tissues
tonsils, pharynx
Describe the MALT
- prominent in mucosal connective tissue ( ie. sub-epithelial lamina propria )
- sites of B-cell and T-cell activation.
- no capsule or afferent lymphatics ( not in the path of lymph-vascular flow)
What are the functions of the spleen?
- RBC destruction
- secondary lymphoid organs
- it has a marginal zone ( macrophages /dendritic cells)
- dendritic cells present antigens to T-cell
- macrophages destroy slow moving /old RBC in red pulp
What are the sites of white pulp in the spleen?
Site of B and T cell activation ( basphillic- lymphocytes)
What are the sites of red pulp in the spleen?
RBC, site of RBC destruction ( eosinophillic)
