Lymphoid System Flashcards
List 5 examples of innate immunity.
- Barriers e.g. skin
- Chemical defence e.g. stomach acid
- Phagocytic cells
- Complement activated system
- Extracellular killers e.g. natural killer lymphocytes
What is a complement activated system?
A cascade of plasma proteins that forms an enzyme system that defend against bacteria
True or False: Adaptive immune system is faster to react than the innate system.
False, it’s slower
What is immune tolerance?
The ability of lymphocytes not reacting to self antigens
What are the primary lymphoid organs?
Bone marrow
Thymus
What are the secondary lymphoid organs?
Spleen
Lymph nodes
MALT tissues
What does MALT stand for?
Mucosal Associated Lymphoid Tissue
What tissues are included in MALT?
Tonsils
Adenoids
Peyer’s patches
Lymphoid aggregates in the intestines
What embryonic tissue is all lymphoid tissue derived from, except for the thymus?
Mesoderm
What is the function of natural killer lymphocytes?
Kill virus-infected cells and some tumour cells
What is the function of B-lymphocytes?
Produce antibodies
What is the function of the 4 types of T-lymphocyte?
Helper: Secreting interleukins that start a cascade response
Cytotoxic: Kill infected cells and cancer cells
Suppressor: Suppress immune response to self antigens and stop an immune response
Memory: Provide a rapid response to the same antigen in the event of reinfection
What does CD stand for?
Cluster designation
Where do the 3 types of lymphocytes develop?
Natural killer and B lymphocytes develop in the bone marrow, while T lymphocytes differentiate and mature in the thymus
Describe the thymus.
2 lobes subdivided by septa
Highly cellular outer cortex
Less cellular inner medulla
Why are the septa of the thymus unusual?
Composed of connective tissue and epithelioreticular cell
Which segment of the thymus stains more intensely with H&E?
Cortex (basophilic)
What processes does thymus involution include?
Fatty infiltration
Lymphocyte depletion
True or False: The thymus continues to provide mature T-cells to the circulation.
True
What secretes thymic hormones?
Cords of epithelial cells in the thymus
What is the epithelial lining type of postcapillary venules at the corticomedullary junction in the thymus?
Specialised cuboidal endothelium that allows the passage of lyphocytes
What is the term for immature and maturing T-cells?
Thymocytes
What happens to T-cells as they move from the cortex to the medulla?
They mature and surface markers are acquired to specialise their role
What is clonal deletion (negative selection) in the thymus medulla?
The destruction of any T-cells that react when presented with self antigens
What are the whorls of epithelial cells with a keratin core in the medulla of the thymus called?
Hassall’s corpuscles
Describe the epithelial cells of the thymic medulla.
Large pale-staining nuclei
Eosinophilic cytoplasm
Prominent basement membranes
What is the function of thymic interdigitating cell?
To present self antigens to maturing T-cells
What is another name for epithelioreticular cells?
Thymic nurse cells
How do reticular cells in the cortex support clusters of maturing lymphocytes?
Cytoplasmic processes linked by desmosomes enclose partially the endothelium of continuous capillaries
What do the blood-thymus barrier do?
Create a physical barrier that protects immature lymphocytes from foreign blood-borne antigens
Describe the contents of endothelioreticular cells.
Lysosomes
Granules
Abundant tonofilaments (intermediate filaments)
How do macrophages support the effectiveness of the barrier?
By engulfing antigens before they can enter the cortex
Why are there only a small number of thymic macrophages in the circulation?
Most of them undergo apoptosis during differentiation
True or False: 400-500 lymph nodes are found in the body.
False, 500-600
Where are lymphoid system cells found?
Connective tissue
What is the function of the lymphatic vessels and associated lymph nodes?
- Drainage of lymph
- Surveillance of tissues for signs of antigens
- Delivery of absorbed fats from the small intestine
Describe lymphatic capillaries morphology.
Blind-ended channels in C.T. spaces
Thin walled capillaries that anastomose and become larger
Anchoring filaments connect endothelial cells to adjacent collagen fibres
What does anchoring filaments in lymphatic capillaries prevent?
Vessel collapse
Where are lymphatic capillaries most abundant?
Dermis
Beneath mucous membranes of respiratory, gastrointestinal and genitourinary tracts
C.T. spaces in the liver
What is the function of lymph vessels?
Absorb interstitial fluid and lymphocytes to add back to the circulation
Where do large lymphatic ducts drain to?
Subclavian vein at the angle junction (where the jugular vein and subclavian vein join)
What embryological tissue do lymph nodes derive from?
Mesenchyme
List the functions of lymph nodes.
- Filtration of lymph
- Production of lymphocytes
- Synthesis of antibodies
- Recirculation of lymphocytes
What is the basic structure of a lymph node?
Capsule
Cortex
Paracortex
Medulla
Subcapsular sinus
Describe the capsule of a lymph node.
Dense fibrous C.T.
Delicate, radiating partitions into the interior of the node (trabeculae)
What is the purpose of lymph node trabeculae?
Provide support
Serve as conduits for blood vessels
What are the most common cells found in lymph nodes?
Lymphocytes
Macrophages
Plasma cells
Reticular cells
Follicular dendritic cells
The lymph node cortex consists of lymphoid nodules. What type of lymphocyte occupies this space?
B-lymphocytes
Name and briefly describe the 2 types of lymph cortical nodules.
Primary: spherical tightly-packed aggregates in a reticular fibre mesh
Secondary: germinal centre surrounded by a mantle zone containing resting B cells
What is a germinal centre in a lymph cortical nodule?
Major sites of B-cell proliferation that contains lymphocytes, lymphoblasts and follicular dendritic cells
If B-cells are found in the cortex of the lymph node, where are T-cells found?
Paracortex (also called thymus-dependent region)
Describe the lymph node medulla.
Pale staining
Next to node hilum
Contains medullary cords
What are medullary cords?
Irregular strands of loosely arranged lymphoid tissue which consist of mainly lymphocytes, macrophages, and plasma cells.
True or False: Medullary sinuses are dilated spaces between medullary cords, frequently bridged by reticular cells and fibres.
True
What are the medullary sinuses continuous with and why?
Cortical sinuses
Because they join at the node hilum to deliver lymph to efferent lymph vessels of the node
What is the subcapsular/marginal sinus?
A narrow channel under the capsule of the lymph node that delivers lymph further into the node
What lines the lymph node sinuses?
Attenuated, discontinuous layer of endothelial cells
What is the function of high endothelial venules?
Allow selective diapedesis of T and B lymphocytes from the blood to perivascular areas
Where are high endothelial venules found in the lymph node?
Paracortex
What is a specialisation of high endothelial venules that aid the function?
Endothelial cells have cell adhesion molecules that allow highly specific migration of T and B lymphocytes
Describe the morphology of high endothelial venules.
Cuboidal endothelial cells
Prominent perivascular sheath
Thick basal lamina
What is homing?
Movement of T and B cells across high endothelial venules into lymph nodes and other sites
What lymphoid organ lacks high endothelial venules?
Spleen
What supports the lymphocyte infiltrations in MALT?
A loose framework of reticular fibres
What are M cells?
Specialised epithelial cells that take up small particles like bacteria so they can be engulfed by submucosal macrophages
Where are M cells abundant?
Dome epithelium of Peyer’s patches
What tissues form Waldeyer’s ring?
Palatine tonsils
Lingual Tonsils
Pharyngeal tonsils
Tubal tonsils (adenoids)
True or False: The luminal surface of palatine tonsils is covered by stratified cuboidal epithelium.
False, stratified squamous epithelium
How are the blind-ended tonsillar crypts formed?
Deep invaginations of the stratified squamous epithelium of the luminal surface
What separates the base of the palatine tonsil from the underlying muscle?
A dense collagenous hemicapsule
What is contained in the tonsillar parenchyma?
Numerous lymphoid follicles with germinal centres
What are the functions of the spleen?
- Produce immune response against blood-borne antigens
- Removal of particulate matter and old blood cells
- Blood cell production during foetal life
Describe the basic structure of the spleen?
Capsule
White pulp
Red pulp
What is a periarteriolar lymphatic sheath?
Islands of lymphoid tissue (white pulp) surrounding a central arteriole
True or False: T lymphocytes are mainly found in the periarteriolar lymphatic sheaths.
True
Where are lymphoid nodules, containing B cells, positioned in the white pulp?
Peripheral white pulp in relation to arterioles
What is the marginal zone of the spleen?
A shell of sparsely cellular lymphoid tissue surrounding the white pulp
What type of pulp makes up most of the spleen?
Red pulp
What does the red pulp consist of?
Venous sinusoids and cords of Billroth
What are cords of Billroth?
Reticular tissue rich in lymphocytes
What is the lining epithelium of venous sinusoids in the spleen?
Longitudinally-oriented endothelial cells with large nuclei that bulge into the sinusoid lumen called stave cells
What is unique about the basement membrane of the layer of stave cells?
Incomplete so flexible cells can pass through
What does the spleen’s marginal zone contain?
Lymphocytes, macrophages and blood antigens
