C8

• Lymphatic system


  Secondary defense system, the primary defense system being the skin and the mucous membranes
• Lymphoid cells, tissues, and organs


  Important components of the immune system that protect the internal environment of the body against any foreign bodies or antigens
• Lymphocytes


  Cells that have the ability to recognize antigens and produce antibodies to react specifically against them
• Components of the lymphoid system

  • Lymphocytes (T- and B-lymphocytes)
  • Lymphoid organs (thymus, spleen, lymph node)
  • Tonsils
  • Lymphatic aggregates in respiratory, digestive and urinary tracts
• Classification of lymphatic tissue

  • Diffuse, unencapsulated lymphatic tissues
  • Dense, unencapsulated lymphatic tissue
  • Dense, encapsulated tissue
• Classification of lymphoid organs

  • Peripheral lymphoid organs (lymph nodes, spleen, tonsils)
  • Central lymphoid organs (thymus, bone marrow, Bursa of Fabricius)
• Cell-mediated immune response


  1. lymphocytes proliferate, attack and directly kill the invading antigens, or release lymphokines to enhance immune response
• Humoral immune response


  1. lymphocytes differentiate into plasma cells and secrete specific antibodies to bind, inactivate, and/or destroy the particular antigen
• Defensive functions of the lymphatic system


  • Production of defensive cells
  • Transport of materials via lymphatic vessels
  • Filtration of lymph and blood through lymphatic organs
  • Phagocytosis and production of hemoglobin
• Cells of the lymphoid system

  • Lymphocytes (T-cells, B-cells, Natural killer cells)
  • Macrophages
  • Plasma cells
  • Reticular cells
  • Antigen-presenting cells
• 1. lymphocytes (T cells)


  Primarily responsible for cell-mediated immunity, carry antibody-like antigen receptors, mature in the thymus gland
• Types of effector T cells

  • Cytotoxic (killer) T cells
  • Helper T cells
  • Suppressor T cells
• 1. lymphocytes (B cells)


  Primarily responsible for humoral immunity, mature in the bone marrow, become activated and differentiate into plasma cells that secrete antibodies
• Natural killer (NK) cells

  Able to attack and lyse invading cells through direct cell-to-cell contact, killing activity is innate and independent of antigenic activation
• Macrophages


  Phagocytose complex antigens and enhance their antigenicity, phagocytose antigen-antibody complexes
• Plasma cells


  Differentiated B-lymphocyte effector cells that secrete immunoglobulins primarily responsible for humoral immunity
• Reticular cells


  Form a meshwork in which lymphocytes, plasma cells, and other tissue components are suspended
• Types of reticular cells

  • Mesenchymal reticular cells
  • Epithelial reticular cells
• Antigen-presenting cells


  Bind antigen-antibody complexes on their surfaces for long periods without phagocytosing them, collect and concentrate antigens for presentation to lymphocytes
• Types of lymphatic nodules

  • Primary lymphatic nodules
  • Secondary lymphatic nodules
• Thymus


  Lympho-epithelial organ that originates from epithelial outgrowths of the third pharyngeal pouch, site of T-lymphocyte production and maturation
• Regions of the thymus


  • Cortex
  • Medulla
• Functions of the thymus

  • T-lymphocyte production
  • Hormone production
  • Immune response
• Effects of thymectomy (removal of the thymus gland in newborns) include lack of immunocompetent T-lymphocytes and early death from infections
• The thymus has a blood-thymus barrier that limits antigenic material from entering the organ
• Epithelial cells of the thymus gland

  Secrete various hormones and humoral growth factors that are believed necessary for proliferation, differentiation, and maturation of B-lymphocytes
• Hormones secreted by epithelial cells of the thymus gland

  • Thymulin
  • Thymopoietin
  • Thymosin-1
  • Thymic humoral factor
• Thymus


  • Important component of the immune system
• Thymectomy (removal of thymus) in newborn

  Lymphoid organs will not become populated by immunocompetent T-lymphocytes, organism does not acquire necessary immunological competence to fight invading pathogens and may die early from complications of infection
• Blood-thymus barrier


  • Three-layered structure (nonfenestrated capillary endothelium, thick basal lamina and reticular cell sheath) that limits the antigenic material to which the thymocytes are exposed in the thymus, helps maintain supply of uncommitted stem cells for later programming during encounters with new antigen
• Adrenocorticosteroids and ACTH


  Slow thymocyte proliferation and reduce the thickness of the thymic cortex
• Androgens and estrogens


  Accelerate thymic involution
• Castration


  Has opposite effects to androgens and estrogens on thymic involution
• Growth hormone


  Stimulates thymic growth in general
• Lymph nodes


  Smallest but most numerous encapsulated lymphoid organs, distributed throughout the body and along the paths of lymphatic vessels, the most organized of all the lymphatic organs and the only one with both the efferent and afferent lymph vessels and sinuses
• Structure of lymph nodes

  • Densed and encapsulated usually bean-shaped structures with convex and concave surfaces, size varies from about 1 mm to several centimeters, parenchyma consists of a peripheral cortex and a central medulla, connective tissue capsule gives off trabeculae that penetrate between the cortical nodules and subdivide the cortex, blood vessels enter and leave through the hilum
• Capsule of lymph nodes

  • Composed primarily of dense irregular collagenous connective tissue, collagen and reticular capsule with a few scattered elastic fibers
• Stroma of lymph nodes


  • Composed of fine reticular cells and fibers that permeate the parenchyma in varying densities, lymphocytes, macrophages and plasma cells are supported by this reticular meshwork
• Sinuses of lymph nodes

  • Lined by endothelial cells that form a continuous lining toward the capsule and trabeculae but often in a discontinuous one toward the parenchyma of the node, lymph percolates into the parenchyma through gaps in the sinus walls, giving the parenchymal cells access to lymph-borne antigens, cells and particulate matter
• Paracortical zone of lymph nodes

  • T-dependent region lying between the cortical lymphoid nodules and the medulla, contains mainly T-lymphocytes suspended in a reticular tissue network, characterized by the presence of high-endothelial post-capillary venules through which T-lymphocytes leave the blood to enter the paracortical zone