(8) LYMPHATIC SYSTEM

• The lymphatic system and the vascular system form an important functional unit called the hemic-lymphatic system.
• The hemiclymphatic system is a secondary defense system and the primary defense system being the skin and the mucous membranes.
• This crucial protective function of the immune system is performed by the lymphocytes because these cells have the ability to recognize antigens; and, by producing antibodies, react specifically against them.
• Antigen is any foreign macromolecule in the body that can provoke an immune response.
• Antibodies are circulating plasma glycoproteins, also called immunoglobulins are specialized to react with the antigens, initiating a complex immune response that protects the body from damage by eventually destroying the foreign substance.
• Antibodies are circulating plasma glycoproteins, also called immunoglobulins are specialized to react with the antigens, initiating a complex immune response that protects the body from damage by eventually destroying the foreign substance.
• lymphoid system’s major functional components are:

  • lymphocytes (T-lymphocytes and B-lymphocytes)
• Bone marrow produces lymphocytes, it can also be considered as lymphoid organ
• Bone marrow produces lymphocytes, it can also be considered as lymphoid organ
• Are important components of the immune system:

  • lymphoid cells
  • tissues
  • organs
• Diffuse, unencapsulated lymphatic tissues are subepithelial lymphatic tissue associated with somatic orifices and tracts of the respiratory, digestive and urogenital systems.
• Diffuse, unencapsulated lymphatic tissues are accumulations of:

  • lymphocytes
  • plasma cells
  • monocytes (histiocytes and macrophages)
• Diffuse, unencapsulated lymphatic tissues are generally found in:

  • lamina propria
• Diffuse, unencapsulated lymphatic tissues - function as a readily available second line of body defense.
• Dense, unencapsulated lymphatic tissue are subepithelial accumulations of lymphatic tissue associated the respiratory, digestive and urogenital tracts.
• Dense, unencapsulated lymphatic tissue consists of solitary lymph nodules with prominent germinal centers.
• Dense, unencapsulated lymphatic tissue occur in:

  • lamina propria,
  • tunica submucosa
  • tunica adventitia (occasionally)
• When solitary lymph nodules are large and become confluent, they are called aggregated lymph nodules.
• Peyer’s patches in the intestinal wall are examples of this aggregated lymph nodules.
• Dense, encapsulated tissue scattered throughout the body are:

  • lymph nodes
  • spleen
  • hemal node
  • hemolymph nodes
  • thymus
  • bursa of Fabricius
• Classification of lymphatic tissue:

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

  • peripheral lymphoid organs
  • central lymphoid organs
• peripheral lymphoid organs:

  • lymph nodes
  • spleen
  • tonsils
• central lymphoid organs:

  • thymus,
  • bone marrow,
  • Bursa of Fabricius in birds
• Two types of closely allied immune responses:

  • Cell-mediated immune response
  • Humoral immune response
• Cell-mediated immune response - the T-lymphocytes proliferate, attack and directly kill the invading antigens while others release lymphokines
• lymphokines - substances that enhance various aspects of immune response
• Cell-mediated immune response - T-lymphocytes may also attack indirectly by activating B-lymphocytes or macrophages.
• Humoral immune response - is the second type of response, in which the B-lymphocyte differentiates into plasma cells and secretes specific antibodies.
• 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
  • Macrophages
  • Plasma Cells
  • Reticular Cells
  • Antigen-Presenting Cells
• T-lymphocytes (T cells) - primarily responsible for cell-mediated immunity that carries antibody-like antigen receptors (but not Igs) on their surfaces.
• T-lymphocytes (T cells) - arise from the lymphocytes that are carried from the bone marrow to the thymus gland.
• Three major types of effector T cells:

  • Cytotoxic (killer) T cells
  • Helper T cells
  • Suppressor T cells
• Cytotoxic (killer) T cells - kill foreign cells such as virus-infected cells, cancer cells and transplants.
• Helper T cells - help B-lymphocytes or T-lymphocytes to respond to antigens and active macrophages.
• Suppressor T cells - suppress B-lymphocytes or T-lymphocytes; they moderate helper cell activity, thereby helping to regulate humoral immune responses.
• B-lymphocytes (B cells) - are primarily responsible for humoral immunity.
• B-lymphocytes (B cells) - they mature and become immunocompetent in the bone marrow.
• When the immunocompetent B-lymphocytes encounter a specific antigen they become activated, proliferate, and differentiate into plasma cell.