2. Lymphoid Tissue Forms the basis of the immune system of the body Forms the basis of the immune system of the body Lymphocyte, the principal cell of lymphoid tissue, is responsible for the proper functioning of the immune system Lymphocyte, the principal cell of lymphoid tissue, is responsible for the proper functioning of the immune system Classified as primary & secondary lymphoid tissue Classified as primary & secondary lymphoid tissue Organized into diffuse and nodular lymphoid tissue Organized into diffuse and nodular lymphoid tissue

3. Types of Lymphoid Organs Primary lymphoid organs are responsible for the development and maturation of lymphocytes into mature cells e.g. fetal liver, bone marrow and thymus Primary lymphoid organs are responsible for the development and maturation of lymphocytes into mature cells e.g. fetal liver, bone marrow and thymus Secondary lymphoid organs are responsible for the proper environment in which the immunocompetent cells can react with each other and with the antigens e.g. lymph nodes, spleen, and mucosa associated lymphoid tissue Secondary lymphoid organs are responsible for the proper environment in which the immunocompetent cells can react with each other and with the antigens e.g. lymph nodes, spleen, and mucosa associated lymphoid tissue

4. Organization of Lymphoid Tissue 1. Diffuse Lymphoid Tissue Occurs throughout the body, especially under wet epithelial membranes where the connective tissue is infiltrated by lymphoid cells e.g. lymphocytes, plasma cells, macrophages and reticular cells Occurs throughout the body, especially under wet epithelial membranes where the connective tissue is infiltrated by lymphoid cells e.g. lymphocytes, plasma cells, macrophages and reticular cells Particularly evident in the lamina propria of GIT & subepithelial connective tissue of the respiratory tract Particularly evident in the lamina propria of GIT & subepithelial connective tissue of the respiratory tract

5. 2. Nodular Lymphoid Tissue Includes: Includes:  Lymph nodes  Thymus  Spleen  Tonsils

6. Cells of Immune System Lymphocytes are generally classified into: Lymphocytes are generally classified into:  B lymphocytes (B cells)  T lymphocytes (T cells)  Null cells: composed of two types  Stem cells  Natural killer cells

7. B Lymphocytes Small lymphocytes that are formed, and become immunocompetent in the bone marrow. Small lymphocytes that are formed, and become immunocompetent in the bone marrow. The activated cells enter the general circulation, undergo mitosis, and transform into antibody-producing plasma cells and B-memory cells The activated cells enter the general circulation, undergo mitosis, and transform into antibody-producing plasma cells and B-memory cells

8. T Lymphocytes Histologically identical to B lymphocytes Histologically identical to B lymphocytes Formed in bone marrow, migrate to the cortex of the thymus to become immunocompetent Formed in bone marrow, migrate to the cortex of the thymus to become immunocompetent There are several subtypes e.g. T helper cells, T cytotoxic cells, T suppressor cells and T memory cells There are several subtypes e.g. T helper cells, T cytotoxic cells, T suppressor cells and T memory cells Assist B lymphocytes in the performance of their functions Assist B lymphocytes in the performance of their functions

9. Antigen Presenting Cells Derived from monocytes Derived from monocytes They phagocytose, catabolize and process antigen and present to T lymphocytes They phagocytose, catabolize and process antigen and present to T lymphocytes Include macrophages, dendritic cells (Langerhans cells of the epithelium & mucosa), B cells and epithelial reticular cells of thymus Include macrophages, dendritic cells (Langerhans cells of the epithelium & mucosa), B cells and epithelial reticular cells of thymus

10. Stem Cells Undifferentiated cells Undifferentiated cells Give rise to the various cellular elements of the blood Give rise to the various cellular elements of the blood

11. Natural Killer Cells Cytotoxic cells Cytotoxic cells Formed in the bone marrow Formed in the bone marrow Become immunocompetent as soon as they are formed Become immunocompetent as soon as they are formed They kill some virally altered cells and tumor cells They kill some virally altered cells and tumor cells

12. Lymph Nodes Ovoid, kidney shaped organs Ovoid, kidney shaped organs Each node has: Each node has:  A convex surface which receives afferent lymph vessels  A hilum where efferent lymph vessels leave and drain lymph from the organs

13. Histological Structure of Lymph Node Each lymph node has a dense irregular collagenous connective tissue capsule Each lymph node has a dense irregular collagenous connective tissue capsule Derived from the capsule, connective tissue septae (trabeculae) extend into the outer part (cortex) of the node and divide it into incomplete compartments Derived from the capsule, connective tissue septae (trabeculae) extend into the outer part (cortex) of the node and divide it into incomplete compartments The framework of the node is formed by reticular connective tissue The framework of the node is formed by reticular connective tissue

14. Each lymph node is divided into three regions: Each lymph node is divided into three regions:  Cortex  Paracortex  Medulla M PC C

15. Lymph Nodes: CORTEX contains the: contains the:  Subcapsular sinus, which is continuous and drains into the  Cortical sinuses  Lymphoid nodules (primary & secondary) composed mainly of B lymphocytes and reticular cells C PC M n c scs cs n

16. Lymph Nodes: PARACORTEX It is the region between cortex and medulla It is the region between cortex and medulla Contains thymus dependent zone mainly T lymphocytes Contains thymus dependent zone mainly T lymphocytes High endothelial venules ( HEV) lined by simple cuboidal epithelium instead of simple squmaus epithelium to facilitate passage of mature T lymphocyte coming from thymus to paracortex of lymph node. High endothelial venules ( HEV) lined by simple cuboidal epithelium instead of simple squmaus epithelium to facilitate passage of mature T lymphocyte coming from thymus to paracortex of lymph node. Has NO nodules Has NO nodules PC C M

17. Lymph Nodes: MEDULLA consists of: consists of:  Medullary cords  Medullary sinusoides. The medullary cords are composed of T lymphocytes, plasma cells and macrophages The medullary cords are composed of T lymphocytes, plasma cells and macrophages The medullary sinusoides are continuous with the subcapsular and cortical sinuses. The medullary sinusoides are continuous with the subcapsular and cortical sinuses. There in no lymphatic Nodules. There in no lymphatic Nodules. MS pc

18. Lymph Nodules Lymph nodules are small masses of lymph tissue in which B lymphocytes are produced. Lymph nodules are small masses of lymph tissue in which B lymphocytes are produced. Lymph nodules may be: Primary nodules Primary nodules Secondary nodules Secondary nodules PN SN

19. Primary nodules which are formed of virgin (not activated) and memory B lymphocytes which are formed of virgin (not activated) and memory B lymphocytes

20. Secondary Nodule Contains: Germinal centres, central light areas filled with immature lymphocytes. Here new lymphocytes proliferate from stem cells that originate in the bone marrow. Germinal centers contains large lymphocytes, mitotic figures, macrophages and plasma cells Germinal centres, central light areas filled with immature lymphocytes. Here new lymphocytes proliferate from stem cells that originate in the bone marrow. Germinal centers contains large lymphocytes, mitotic figures, macrophages and plasma cells The germinal centre is surrounded by a darker- staining region called the corona that contains small B lymphocytes ( not activated). The germinal centre is surrounded by a darker- staining region called the corona that contains small B lymphocytes ( not activated). c

21. Lymph Flow Through The Lymph Node Afferent lymph vessels Subcapsular sinuses Cortical sinuses Medullary sinuses Efferent lymphatic vessels

22. Lymph Node: Function Maintenance and production of immunocompetent cells Maintenance and production of immunocompetent cells Proliferation of lymphocytes. Activated T- and B- lymphocytes multiply in the lymph nodes Proliferation of lymphocytes. Activated T- and B- lymphocytes multiply in the lymph nodes Filtration of lymph ( not blood) Filtration of lymph ( not blood) Phagocytosis Phagocytosis

23. Thymus Bilobed lymphoid organ located in thorax Bilobed lymphoid organ located in thorax Derived from endoderm Derived from endoderm Enclosed in a thin connective tissue capsule Enclosed in a thin connective tissue capsule Septae extend from the capsule into the organ, incompletely subdividing it into lobules Septae extend from the capsule into the organ, incompletely subdividing it into lobules Possesses no lymph nodules Possesses no lymph nodules L L S C

24. Thymus cont’d Each lobule is divided into an outer cortex and inner medulla Each lobule is divided into an outer cortex and inner medulla CORTEX: Composed of epithelial reticular cells, macrophages, and thymocytes (small T lymphocytes). Here the immature T cells undergo proliferation, reticular cells release hormone and factor to activate T lymphocyte and transform into immunocompetent cells and then migrate to medulla CORTEX: Composed of epithelial reticular cells, macrophages, and thymocytes (small T lymphocytes). Here the immature T cells undergo proliferation, reticular cells release hormone and factor to activate T lymphocyte and transform into immunocompetent cells and then migrate to medulla MEDULLA: consists of epithelial reticular cells, large T lymphocytes and thymic (Hassall’s) corpuscles MEDULLA: consists of epithelial reticular cells, large T lymphocytes and thymic (Hassall’s) corpuscles Microphage will engulf not activated T lymphocyte. Microphage will engulf not activated T lymphocyte. 1 - cortex 2 - medulla 3 - Hassal's corpuscle 4 - interlobular connective tissue (septa)

25. Hassall’s Corpuscle Are composed of groups of keratinized epithelial cells Are composed of groups of keratinized epithelial cells Increase in number with age Increase in number with age Probably represent a degenerative phenomenon Probably represent a degenerative phenomenon CM HC

26. Epithelial Reticular Cells Separate the cortex from the capsule and the trabeculae Separate the cortex from the capsule and the trabeculae Surround the cortical capillaries thus forming a blood-thymus barrier Surround the cortical capillaries thus forming a blood-thymus barrier Form a specialized barrier between cortex & medulla, preventing medullary material from gaining access to the cortex Form a specialized barrier between cortex & medulla, preventing medullary material from gaining access to the cortex Thus provide an antigen free environment for the potentiation of the immunocompetent T lymphocytes Thus provide an antigen free environment for the potentiation of the immunocompetent T lymphocytes They also form Hassall’s (thymic) corpuscles They also form Hassall’s (thymic) corpuscles

28. Thymus

30. Lymph Node