Term
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Definition
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Term
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Definition
| dense connective tissue, continues into organ as trabeculae |
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Term
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Definition
entry and exit site of blood vessels and site of exit of efferent lymphatics
- afferent lymphatics enter everywhere on the surface of node except at hilus |
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Term
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Definition
- reticular fibers associated with reticular cells
- cells adherent to trabeculae and fibers form a loose, open network of sinuses
- open spaces within stroma contain lymphocytes, plasma cells, macrophages |
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Term
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Definition
- route of lymph flow through lymph node: afferent lymphatic to subcapsular sinus to cortical and trabecular sinuses, to medullary sinuses (are relatively large) to efferent lymphatics
- are interconnected
- lined by squamous, endothelium-like cells with processes that extend to one another and phagocytic cells (macrophages)
- the latter cell more numerous in medullary sinuses |
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Term
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Definition
| a. divided into two "layers" - not morphologically delineated |
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Term
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Definition
composed of nodular masses of lymphocytes
- primary nodules (or follicles) dense aggregates of small lymphocytes, are mostly B lymphocytes. The B lymphocytes are predominantly naïve lymphocytes and some memory cells.
- secondary nodules (or follicles): dense aggregates of small lymphocytes (mostly B) surrounding a paler, central area called the germinal center |
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Term
| c. Germinal Center: organization |
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Definition
- predominantly activated B lymphocytes engaged in production of antibodies
- arise because of antigen-stimulated high rate of mitotic activity
- high rate of mitotic activity; hence are "germinal centers"
- B cells are in various stages of maturation
- contain some T cells and macrophages
- contain some antigen presenting cells (APC), some of the latter are stellate shaped with many processes, termed "dendritic cells." Trap antigen for processing and presentation to lymphocytes
- B cells initiate their development into plasma cells in the germinal center but move to the medulla before differentiation is complete. Differentiation is finalized in the medulla.
- germinal center generates an expanded population of identical cells that recognize the same antigen
- generates a clone of cells
- cells of germinal center characterized by having more open nuclei and more cytoplasm. Hence are medium and large lymphocytes. These cells are less densely packed than those in the periphery of the nodule.
- germinal centers are physiologic; arise and disappear
- germinal centers don't appear till after birth due to absence of antigens to stimulate their formation
- germ-free environment animals do not have germinal centers in their lymphoid nodules |
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Term
| d. Deep cortex (paracortical area) |
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Definition
dense but not nodular collections of lymphocytes, subadjacent to outer cortical nodular lymphoid tissue
- populated by recirculating lymphocytes; reach this area by emigrating out of hematogenous capillaries with tall endothelium (these epithelial cells have special surface markers that circulating lymphocytes recognize)
- small lymphocytes predominate, mostly T lymphocytes
- also present: some dendritic antigen presenting cells (most APC's are a special form of macrophage, phagocytose antigenic material, process it and present the fragments to lymphocytes) and other macrophages. These cells not usually readily identified in routine H&E stained preparations
- 70–90% of all lymphocytes entering a lymph node arrive via the "high endothelial venules" |
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Term
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Definition
a. medullary cords: aggregations of lymphoid cells arranged around small blood vessels, branch and anastamose
- open network of sinuses separate the medullary cords from one another
- composition of medullary cords: small lymphocytes are most common cell, plasma cells are particularly numerous if the node is stimulated. Macrophages, reticular cells, reticular fibers
- most lymphocytes here are B lymphocytes |
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Term
| Circulation of Blood in Lymph Node |
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Definition
- arteries enter hilus run within trabeculae - enter medullary cords - reach cortex - break up into rich capillary network - form postcapillary venules with cuboidal or columnar epithelium in deep cortex - continue as morphologically normal veins through medulla and exit the lymph node at the hilus
- postcapillary venules: site of migration of T and B lymphocytes, out of blood vessel into parenchyme of node (similar vessels observed in area of Peyer's patches, thymic medulla, appendix, tonsils)
- selectins (surface molecules on a lymphocyte) allow a cell to recognize and bind to the markers on the surface of tall endothelium. This permits their migration through the wall of the post capillary venule. |
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Term
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Definition
A. Lymph Nodes (as well as thymus, spleen, tonsils) regress in size with age lymph nodes retain ability to enlarge throughout life (e.g. sore throat, infectious mononucleosis)
B. Allow interaction between antigen and lymphoid cells
C. Main site for expansion (increase of number) of lymphocytes in an immune response
D. Production of antibodies
E. Filter (trap) foreign material, bacteria, tumor cells |
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Term
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Definition
A. Size: - largest lymphoid organ, 150 gm
- Role: - is interposed in blood stream (as opposed to lymph nodes which are associated with lymph flow)
- best understood as a discriminatory filter with specialized blood flow |
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Term
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Definition
dense, connective tissue, minimal amount of smooth muscle and elastic fibers
- trabeculae - project throughout the organ
- hilus - site of entry and exit of splenic blood vessels
- reticular fibers and reticular cells comprise the stroma; fibers are continuous with capsule/trabeculae |
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Term
| C. Macroscopic appearance of cut surface of fresh spleen |
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Definition
1. red pulp
- looks dark red due to large amount of blood
2. white pulp
- appears as tiny grey-white dots approx. 0.5 mm in diameter; are the splenic nodule
- approximately 20% of normal spleen |
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Term
| C. Macroscopic appearance of cut surface of fresh spleen |
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Definition
1. red pulp
- looks dark red due to large amount of blood
2. white pulp
- appears as tiny grey-white dots approx. 0.5 mm in diameter; are the splenic nodule
- approximately 20% of normal spleen |
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Term
| D. White Pulp - Microscopic Architecture |
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Definition
- aggregates of lymphocytes: both dense non-nodular collections as well as nodules; typically can see an artery within the lymphoid tissue. It is termed the central artery or the artery with a lymphoid sheath.
- nodular lymphoid accumulations represent concentrations of B lymphocytes; may develop germinal centers
- in humans germinal centers develop within 24 hr after exposure to antigens
- dense non-nodular aggregations of lymphocytes surrounding the central artery are mostly T lymphocytes
- i.e., can identify patches of nodular and dense non-nodular collections of lymphocytes.
- the lymphoid sheaths surround the arterioles as soon as they leave the trabeculae to enter the organ stroma
- the lymphoid sheaths follow the blood vessels until almost capillary size |
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Term
| D. White Pulp - Microscopic Architecture |
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Definition
- aggregates of lymphocytes: both dense non-nodular collections as well as nodules; typically can see an artery within the lymphoid tissue. It is termed the central artery or the artery with a lymphoid sheath.
- nodular lymphoid accumulations represent concentrations of B lymphocytes; may develop germinal centers
- in humans germinal centers develop within 24 hr after exposure to antigens
- dense non-nodular aggregations of lymphocytes surrounding the central artery are mostly T lymphocytes
- i.e., can identify patches of nodular and dense non-nodular collections of lymphocytes.
- the lymphoid sheaths surround the arterioles as soon as they leave the trabeculae to enter the organ stroma
- the lymphoid sheaths follow the blood vessels until almost capillary size |
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Term
| E. Red pulp - Microscopic Architecture |
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Definition
1. Composed of reticular framework containing two major components, splenic sinuses and splenic cords
- splenic sinuses are venous sinuses filled with blood cells (erythrocytes and leukocytes)
- thin areas between the sinuses are called splenic cords; this area contains all the cells of the circulating blood as well as some others
2. Sinuses - irregular, tortuous, anastomosing channels
- lined by fusiform endothelial cells oriented parallel to long axis of sinus
- endothelial cells separated by narrow intercellular clefts
- blood cells pass through the slits in between parallel adjacent endothelial lining cells
- endothelial cells lie on a discontinuous basal lamina causing the latter to form rings or hoops which encircle the sinus (like the metal rings that encircle the boards “the staves” of a barrel)
3. Splenic cords (of Billroth)
- are intersinusoidal spaces
- contain reticular cells and fibers, erythrocytes, macrophages, plasma cells, lymphocytes, platelets; all the cells of circulating blood
- macrophages may contain pigment (hemosiderin) from phagocytosed senescent erythrocytes
- relatively few T and B lymphocytes, most are said to be diverted from the cords to white pulp
- splenic cords can be considered a part of the vascular pathway interposed between the end of the arteries and the sinuses
4. Marginal zone
- is a 80-100 µm wide zone of red pulp immediately adjacent to the white pulp
- can be considered a structurally transitional region between the proper white and red pulp
- contains smaller venous sinuses
- contains dispersed lymphocytes, plasma cells, macrophages
- its importance: is the site where lymphocytes of the recirculating pool leave the blood to enter the white pulp (the lymphoid tissue surrounding the central ateries, the periarterial lymphoid sheaths)
- lymphocytes traversing this region come in contact with dendritic antigen-presenting cells which offer antigen to them |
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Term
| F. Blood flow in the spleen |
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Definition
1. Splenic artery (hilus) divides and enters into trabeculae as trabecular arteries, then enter lymphoid sheaths (white pulp) as central arteries. These arteries have a periaterial lymphoid sheath (PALS)
2. Some branches from central artery supply lymphoid tissue while others travel to periphery of sheath and terminate in marginal zone.
3. The central artery when reduced to 40–50 μ m diameter leaves the white pulp and enters the red pulp.
4. On entering the red pulp, the former central artery divides into several smaller vessels called penicillar (penicillary) arterioles.
5. These arteries divide into 2–3 smaller branches.
6. How or where do the final branches of the central artery (i.e. the penicillary art.) terminate?
- Is not readily obvious under the microscope
- The arterial capillary releases its blood into the pulp cord and the blood cells enter the sinuses by passing in between the endothelial cells that make up the sinus wall.
7. thus the blood reaches the sinus and exits it via a short pulp vein that is connected to the sinus.
8. These veins enter trabeculae to become trabecular veins which in turn join to become the splenic vein which leaves the organ. |
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Term
| G. Functions of the spleen |
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Definition
1. fetal: erythropoiesis, leukopoiesis, megakaryopoiesis
2. filters antigens in the circulating blood, antigen presentation by antigen presenting cells and initiation of immune response
3. production of lymphocytes and related immune functions
4. destruction of aged, abnormal or damaged eythrocytes
5. pitting function of spleen
6. storage and destruction of platelets
7. sequesters monocytes to become macrophages
8. signs of splenectomy
- jolly bodies in erythrocytes of circulating blood
- increased number of damaged red cells in blood
- increased numbers of platelets in blood(i.e. decreased destruction or sequestration) |
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Term
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Definition
| - the transfer of antigens from the mucous membrane to the lymphoid cells of MALT (mucosal-associated lymphoid tissue) is carried out by these specialized cells |
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Term
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Definition
- Initial embroyonic source of cells that colonize the thymus not yet finalized; after birth by cells from bone marrow
- First lymphoid organ to be populated by lymphocytes
- Considered a primary lymphoid organ and source of T lymphocytes while spleen and lymph nodes ect are secondary lymphoid organs. (the other primary lymphoid organ is the bone marrow and is the primary source of B lymphocytes).
B. Growth pattern - size
- relatively largest in neonatal life and birth |
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Term
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Definition
- Initial embroyonic source of cells that colonize the thymus not yet finalized; after birth by cells from bone marrow
- First lymphoid organ to be populated by lymphocytes
- Considered a primary lymphoid organ and source of T lymphocytes while spleen and lymph nodes ect are secondary lymphoid organs. (the other primary lymphoid organ is the bone marrow and is the primary source of B lymphocytes).
B. Growth pattern - size
- relatively largest in neonatal life and birth |
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Term
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Definition
1. microscopic appearance:
- darkly staining due to large number of lymphocytes
- lymphocytes are not arranged in nodules, no germinal centers
2. stroma: epithelial reticular cells, (i.e. epithelial cells derived from endoderm of pharyngeal pouches, an appearance similar to reticular cells/fibroblasts
- morphology of epithelial reticular cells is very variable.
- in the deep cortex the epithelial reticular cells are stellate in shape forming extensive network of intercellular spaces which become full of developing lymphocytes
- major function of cortical epithelial reticular cells is to isolate the developing lymphocytes
- epithelial reticular cells difficult to identify in cortex (obscured by the lymphocytes)
3. Cellular population: lymphocytes, epithelial reticular cells, some macrophages
- mostly T lymphocytes
- a few B lymphocytes also present
- dendritic cells |
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Term
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Definition
1. microscopic appearance:
- darkly staining due to large number of lymphocytes
- lymphocytes are not arranged in nodules, no germinal centers
2. stroma: epithelial reticular cells, (i.e. epithelial cells derived from endoderm of pharyngeal pouches, an appearance similar to reticular cells/fibroblasts
- morphology of epithelial reticular cells is very variable.
- in the deep cortex the epithelial reticular cells are stellate in shape forming extensive network of intercellular spaces which become full of developing lymphocytes
- major function of cortical epithelial reticular cells is to isolate the developing lymphocytes
- epithelial reticular cells difficult to identify in cortex (obscured by the lymphocytes)
3. Cellular population: lymphocytes, epithelial reticular cells, some macrophages
- mostly T lymphocytes
- a few B lymphocytes also present
- dendritic cells |
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Term
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Definition
1. stroma: epithelial reticular cells
- are more prominent than in the cortex
- pale-staining nucleus, acidophilic cytoplasm
- one version of epithelial reticular cells become flattened, are concentrically arranged and undergo keratinization. These structures termed Hassall's corpuscles (thymic corpuscles).
- Function of Hassell’s corpuscles: not fully understood but are functionally active. They produce thymic hormones (e.g. thymosin, thymopoietin).
2. Cellular population: lymphocytes, epithelial reticular cells, dendritic cells
- lymphocytes fewer in number than in cortex, hence medulla stains lighter |
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Term
| E. Development of lymphocytes in thymus |
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Definition
1. thymus is the source of T lymphocytes
2. the site of production of T lymphocytes
3. undifferentiated lymphoid cells from bone marrow arrive at the thymus
4. undergo intensive proliferation in the cortex
5. lymphocytes of outer cortex described as large and immature (difficult to verify)
6. as these thymic lymphocytes (T lymphocytes, T cells) mature they are pushed (or migrate towards the medulla)
7. the mature lymphocytes are small
8. clones of T cells are produced
9. the lymphocytes enter the medulla, continue their maturation, and leave the thymus via blood vessels at the cortico-medullary junction
10. go to peripheral lymphoid tissues and attain full immunologic maturity
11. many lymphocytes of the inner cortex and medulla have a pyknotic nucleus and are probably degenerating. Only a small percentage of the T cells complete their differentiation and leave to populate other organs. Cells having the potential of anti-self activity are prevented from maturing and allowed to degenerate ("clonal deletion"). Estimate 95% of lymphocytes undergo programmed cell death. |
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Term
| F. Functions of epithelial reticular cells |
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Definition
1. close contacts between the reticular cells and the lymphocyte is required for the latter to acquire the capacity to distinguish between "self and non-self."
2. secrete thymotaxin a chemotactic peptide that induces lymphoblasts to leave the blood and enter the cortex of the thymus
3. secrete thymulin: stimulates immature T lymphocytes to synthesize their surface markers
4. produce hormones and products that not only regulate maturation and proliferation of T lymphocytes within the thymus but also distant lymphoid organs. |
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Term
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Definition
1. arteries enter via interlobular septa, reach the cortico-medullary junction. These give rise to:
- capillaries that enter cortex
- arterioles that enter medulla
2. capillaries in cortex are not fenestrated, have a prominent basal lamina and are surrounded by a sheath of reticular cells that also exhibit a marked basal lamina Purpose of this "blood-thymus barrier?" presumably selective transfer of material from blood
3. cortical capillaries reach outer cortex, return to cortico-medullary boundary and join postcapillary venules with high endothelium (the locus of transcapillary entrance of lymphocytes into the thymus)
4. medullary arterioles yield fenestrated capillaries, leading to venules which join the venous vessels at the cortico medullary boundary and leave the organ
H. Involution of thymus
- initiates at puberty
- cortical and medullary concentration of lymphocytes decreases
- fat cells appear
- cortico-medullary distinction disappears |
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Term
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Definition
A. location: in or near pharynx
- classification: palatine, lingual, pharyngeal
B. Morphology
- all lie under a surface layer of epithelium
- epithelial depressions called "crypts" penetrate into the lymphoid tissue
- epithelium typically infiltrated with lymphocytes; source of leukocytes in saliva
- lymphoid tissue consists of collections of lymphoid nodules in diffuse aggregates of lymphocytes, germinal centers present |
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Term
| Development of lymphocytes |
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Definition
A. lymphoid stem cell gives rise to mature, typically small lymphocytes as seen in blood or periphery of a lymphoid nodule. Typical sequence: the large lymphocyte of tissue sections (lymphoblast) undergoes mitosis to give rise to medium size lymphocytes and these, in turn, undergo mitosis and develop into mature small lymphocytes.
B. The small lymphocyte is not an end (post-mitotic) cell! (at least some lymphocytes)
1. upon stimulation the small lymphocyte undergoes blastogenic transformation, increases in size, nucleus assumes a young appearance, becomes an immature blast and undergoes mitosis, to yield a number of identical cells; "blastogenesis"
- synonyms: immunoblast, activated lymphocyte |
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Term
| Development of lymphocytes |
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Definition
A. lymphoid stem cell gives rise to mature, typically small lymphocytes as seen in blood or periphery of a lymphoid nodule. Typical sequence: the large lymphocyte of tissue sections (lymphoblast) undergoes mitosis to give rise to medium size lymphocytes and these, in turn, undergo mitosis and develop into mature small lymphocytes.
B. The small lymphocyte is not an end (post-mitotic) cell! (at least some lymphocytes)
1. upon stimulation the small lymphocyte undergoes blastogenic transformation, increases in size, nucleus assumes a young appearance, becomes an immature blast and undergoes mitosis, to yield a number of identical cells; "blastogenesis"
- synonyms: immunoblast, activated lymphocyte |
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Term
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Definition
A. Generation of immunity
1. two forms:
a. humoral immunity: manifested by production of antibodies (glycoproteins) by plasma cells, quantifiable as immunoglobulins IgG, IgM, etc. are identifiable in serum
b. production of cellular immunity:
- includes specific reactions mediated (performed) by certain lymphocytes, e.g. homograft rejection |
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Term
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Definition
A. Generation of immunity
1. two forms:
a. humoral immunity: manifested by production of antibodies (glycoproteins) by plasma cells, quantifiable as immunoglobulins IgG, IgM, etc. are identifiable in serum
b. production of cellular immunity:
- includes specific reactions mediated (performed) by certain lymphocytes, e.g. homograft rejection |
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Term
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Definition
A. Two broad categories of lymphocytes in man
1. is based on a model found in birds
2. birds have two primary sites where lymphocytes develop first: the Thymus and Bursa of Fabricius, (hence T and B lymphocytes)
3. Bursa of Fabricius - a lymphoid organ off the cloaca (a region in the hindgut of birds)
4. these two sites give rise to lymphocytes which then migrate to all other organs of the body; each lymphocyte population has different functions in generation of immunity
B. Removal of Bursa of Fabricius (in embryo, at birth or soon thereafter)
1. reveals that the organ is essential for humoral immunity
2. leads to absence of: plasma cells, immunoglobin production and germinal centers in lymphoid nodules of spleen, lymph nodes, etc.
C. Removal of thymus (neonatal thymectomy in mice)
1. low number of circulating lymphocytes, depletion of T lymphocytes in spleen and lymph nodes
2. impaired cellular immune reactions result such as delayed graft rejection
3. B lymphocyte pool remains normal but fails to be stimulated by most antigens due to lack of appropriate message from T cells
D. Bursa of Fabricius in man?
1. man does not have one
2. nevertheless, man has T & B lymphocytes
3. bursal equivalent site in man: bone marrow |
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Term
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Definition
A. Two broad categories of lymphocytes in man
1. is based on a model found in birds
2. birds have two primary sites where lymphocytes develop first: the Thymus and Bursa of Fabricius, (hence T and B lymphocytes)
3. Bursa of Fabricius - a lymphoid organ off the cloaca (a region in the hindgut of birds)
4. these two sites give rise to lymphocytes which then migrate to all other organs of the body; each lymphocyte population has different functions in generation of immunity
B. Removal of Bursa of Fabricius (in embryo, at birth or soon thereafter)
1. reveals that the organ is essential for humoral immunity
2. leads to absence of: plasma cells, immunoglobin production and germinal centers in lymphoid nodules of spleen, lymph nodes, etc.
C. Removal of thymus (neonatal thymectomy in mice)
1. low number of circulating lymphocytes, depletion of T lymphocytes in spleen and lymph nodes
2. impaired cellular immune reactions result such as delayed graft rejection
3. B lymphocyte pool remains normal but fails to be stimulated by most antigens due to lack of appropriate message from T cells
D. Bursa of Fabricius in man?
1. man does not have one
2. nevertheless, man has T & B lymphocytes
3. bursal equivalent site in man: bone marrow |
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Term
| Comparison of T and B lymphocytes in man |
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Definition
A. under light microscopy have identical appearance; no routine staining differentiates between T and B cells
- the small lymphocytes of blood are both T and B lymphocytes
- T and B lymphocytes are identifiable by immunocytochemical methods (cell markers) |
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Term
| Comparison of T and B lymphocytes in man |
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Definition
A. under light microscopy have identical appearance; no routine staining differentiates between T and B cells
- the small lymphocytes of blood are both T and B lymphocytes
- T and B lymphocytes are identifiable by immunocytochemical methods (cell markers) |
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