Lymphatic "Immune" System PDF

LYMPHATIC "IMMUNE" SYSTEM Cells Lymphatic organs Lymphocytes APC capsulated non-capsulated ANTIGEN PRESENTING CELLS "APC":  Origin: bone marrow.  Site: scattered in many organs as skin "not...

LYMPHATIC "IMMUNE" SYSTEM Cells Lymphatic organs Lymphocytes APC capsulated non-capsulated ANTIGEN PRESENTING CELLS "APC":  Origin: bone marrow.  Site: scattered in many organs as skin "not only in lymphatic organs".  Group of cells that phagocytose the antigen.  Intracellular partial processing of the antigen occurs.  Then they express the most antigenic part of the antigen “epitope” on their surface attached to MHC II receptors and present this complex to T-helper cell … why?  T cell receptors can only interact with cells that have this combination of MHC II U U and the antigen.  They have ABUNDANT lysosomes. U U  These cells are: 1. Macrophages. 2. Reticular cells. 3. Dendritic cells in lymph nodes and spleen. 4. Langerhans cells. 5. B lymphocytes. NON-CAPSULATED LYMPHOID TISSUES:  Site: Present in the corium of the mucous membranes of the GIT or the bronchial tree.  Structure: 1. Stroma: formed of o Reticular fibers. o Dendritic reticular cells “secrete reticular fibers and are APC”. 2. Parenchyma mainly follicles formed of: o B-lymphocytes mainly. o T-lymphocytes. o Macrophages. o Plasma cells.  At the periphery of the follicles there is a special type of venule called high U endothelial venule: U - Lined by cubical cells. - These cells has homing receptors on their surface to address lymphocytes to their specific location.  They are NOT provided by afferent lymphatics.  They has efferent lymphatics to the regional lymph nodes. SOLITARY LYMPHOID FOLLICLE:  Solitary.  Temporary.  Under mucous membranes.  Formed in response to entry of an antigen.  Structure: same above…  It is oval or rounded mass of cells with deeply stained center and periphery “primary lymphatic follicle”.  It may show germinal center “secondary lymphatic follicle”. Secondary lymphatic follicle: U  Spherical mass of cells with dark periphery and pale center called germinal center. U U  It is formed in response to antigenic stimulation.  Lymphocytes are activated, increased in size with abundant cytoplasm and vesicular nuclei. They differentiate plasma cells.  Cells of the germinal center: - Activated B-lymphocytes. - Plasma cells. - Follicular dendritic cells. - Macrophages. - T- helper cells. AGGREGATED LYMPHOID TISSUE  Site: in the corium of the mucous membranes "Mucosal associated Lymphatic Tissue MALT".  Multiple.  Permanent. U  Present in the corium of the gut “GALT, Gut Associated Lymphatic Tissue” which are: 1. Peyer’s patches of the ileum. 2. Tonsils. 3. Appendix.  Present in the corium of the bronchial tree “BALT Bronchus Associated Lymphatic Tissue”: protect the respiratory tract (BALT).  Structure: same as above…  It shows germinal center if it is active. THYMUS  Bilobed organ in the anterior mediastinum.  It has dual lymphoepithelial origin. 1. Endoderm: Epithelial reticular cell. 2. Mesoderm: T-lymphocytes.  It is large early in life and involutes near puberty.  The thymus varies with age: - Prominent in newborns. - Size increases in childhood. - Growth stops during adolescence. - It atrophies in adulthood. - By old age it has been largely replaced by fibrous and fatty connective tissue. STRUCTURE: Stroma:  Capsule: dense irregular C.T.  Septa: Arise from the capsule. They are thin incomplete septa dividing the thymus into incomplete lobules.  NO RETICULAT FIBERS in the thymus.The reticular framework that supports the parenchyma is formed by the processes of reticular epithelial cells "CYTORETICULUM". U Parenchyma: Thymic lobules: Each lobule is formed of: 1. Cortex. 2. Medulla. THYMIC CORTEX:  It is the outer dark region of the lobule.  It is the site of maturation of T-lymphocytes. U U  It contains: 1. T- lymphoblast. 2. T-lymphocytes. 3. Epithelial reticular cells. 4. Macrophages. Thymocytes "T-lymphoblast":  Origin: mesoderm. They are derived from the bone marrow.  They are incompetent cells. U U  Surrounded by the processes of epithelial reticular cells separating them from antigens during their maturation.  As they mature, they migrate towards the deeper part of the cortex and then to the U U medulla (most T cells die by apoptosis).  Mature “immunocompetent” cells migrate to medulla and enter blood stream U U U U through postcapillary venule at the corticomedullary junction to settle in the thymus U dependent zones of lymph nodes & spleen: U 1. Paracortex of lymph node. 2. Periarteriolar lymphatic sheath of the spleen. Epithelial reticular cell "ERC":  Endodermal in origin.  They are all 6 types "from I to VI":  Three types in the cortex: I, II and III.  With LM: o Stellate in shape. o Pale basophilic cytoplasm. o Large ovoid vesicular central nucleus.  With EM: o Long processes filled with tonofilaments. o The processes are joined by occluding junctions. U U o They have: - Ribosomes. - rER. - Golgi apparatus. - Secretory granules.  Function: o They create the special microenvironment for the developing T-lymphocytes “NURSING" cells.  o They synthesize thymic hormones (thymosin and Thymopoietin H, serum thymic factor). o They prevent contact between T-cells and blood born antigens "blood thymic barrier" “Blood Thymic Barrier” BLOOD THYMIC BARRIER:  Exists in the cortex making it an immunologically protected region.  Formed of: 1. Endothelium of thymic capillaries (continuous type). 2. Complete thick basal lamina. 3. Perivascular C.T. and macrophages. 4. Complete layer of ERC (Type I).  Function: prevent escape of antigens from blood to the developing T-lymphocytes in the thymic cortex. THYMIC MEDULLA:  It is the central pale area of the thymus lobule.  It is the site where T-lymphocytes leave the thymus.  It contains: 1. Epithelial reticular cells (Types IV, V, VI). 2. Mature T-lymphocytes. U U 3. Macrophages. 4. Thymic or Hassall’s Corpuscle: o It is present only in the medulla. U U o Formed of concentric layers of ERC (type VI) around central acidophilic mass. o Formed due to degeneration of ERC. o They increase in number with age. o Unknown function "may be the site of TL apoptosis". Function of the thymus: U  Proliferation, maturation and differentiation of T-cells in the cortex.  Thymic reticular epithelial cells secrete several hormones or factors which stimulate the differentiation of T-lymphocytes Medical application: Acquired immune deficiency syndrome AIDS: the virus infects and destroys T helper cells. The patients become easily susceptible to infections. DiGeorge syndrome: failure of development of the third and fourth pharyngeal pouches leads to thymic hypoplasia. This leads to lack of all thymic epithelial cells with improper development of T lymphocytes leading to severe depression of cell-mediated immune response.  TONSILS  The tonsils are aggregates of lymphoid tissue  The tonsils are named according to location: 1. Palatine tonsils are located on the lateral wall of the oropharynx. 2. The lingual tonsils lie at the base of the tongue. 3. The pharyngeal tonsils are found on the posterior wall of the nasopharynx. Palatine tonsil:  Two tonsils on the lateral wall of the oropharynx.  It consists of: 2. Stratified squamous epithelium non-keratinized covering it with crypts “primary and secondary”. 3. Lymphoid tissue: in the form of - Lymphoid follicles. - Diffuse lymphatic tissue. 4. Incomplete capsule with mucous gland outside the capsule. Pharyngeal tonsil:  Single on the posterior wall of the nasopharynx.  It consists of: 1. Pseudo stratified columnar ciliated epithelium with goblet cells. 2. It has folds no crypts. 3. Lymphoid tissue: in the form of - Lymphoid follicles. - Diffuse lymphatic tissue. 4. Seromucous glands open in the folds covered by incomplete capsule.  When infected → adenoids. Lingual tonsil:  Multiple small masses at the base of the tongue.  It consists of: 1. Stratified squamous epithelium non-keratinized covering it. 2. Single wide non branching crypt. 3. Seromucous glands open into the crypt Function of the tonsils: Provide the body with defense against air born or food born U U antigens. Compare the microscopic structure between thymic cortex and medulla Thymic cortex Thymic medulla Type of T cell T lymphoblast “ immature” T lymphocyte “mature” Type of epithelial reticular cell Type I, II and III Type IV, V and VI Blood thymic barrier present Absent Hassel corpuscle absent Present Function T cell maturation and T cell exit to the blood development Compare the three types of tonsils Palatine tonsil Pharyngeal tonsil Lingual tonsil Number Two Single Multiple Overlying Stratified squamous non Pseudostratified columnar Stratified squamous non epithelium keratinized ciliated with goblet cells keratinized Crypt Primary and secondary No crypts but folds Single unbranched crypts crypts Define blood thymic barrier and list its structures BLOOD THYMIC BARRIER:  Exists in the cortex making it an immunologically protected region.  Formed of: 1. Endothelium of thymic capillaries (continuous type). 2. Complete thick basal lamina. 3. Perivascular C.T. and macrophages. 4. Complete layer of ERC (Type I).  Function: prevent escape of antigens from blood to the developing T-lymphocytes in the thymic cortex. Give reasons for:  Thymocytes "T lymphoblast" are protected from blood born antigen during their development in the thymus This is because: 1. The thymus has NO afferent lymphatics so, no lymph born antigens U U 2. The presence of blood thymic barrier which protects the developing T lymphocytes from the blood born antigens