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University of Toronto Scarborough
Biological Sciences
Kenneth Welch

The Lymphatic System and Immunity An Overview of the Lymphatic System and Immunity lymphatic system •The lymphatic system •Contains cells, tissues, and organs responsible for defending the body •Lymphocytes resist infection and disease by responding to •Invading pathogens such as bacteria or viruses •Abnormal body cells such as cancer cells •Foreign proteins such as toxins Organization of the Lymphatic System The lymphatic system consists of •Lymph •Lymphatic vessels •Lymphoid tissues and organs •Lymphocytes and supporting phagocytic cells Functions of lymphatic system •Primary function is production, maintenance, and distribution of lymphocytes •Lymphocytes must: •Detect where problems exist •Be able to reach the site of injury or infection Lymphatic vessels include •Lymphatic capillaries •Small lymphatic vessels •Major lymph-collecting vessels Major lymph-collecting vessels •Superficial and deep lymphatics •Thoracic duct •Cisterna chyli •Right lymphatic duct Lymphocytes •Three classes of lymphocytes •T (thymus dependent) cells •B (bone marrow-derived) cells •NK (natural killer) cells Lymphocyte production (lymphopoiesis) •Involves bone marrow, thymus, and peripheral lymphoid tissue •B cells and NK cells mature in bone marrow •T cells mature in the thymus Lymphoid tissue •Connective tissue dominated by lymphocytes •Lymphoid nodules •Lymphocytes densely packed in areolar tissue •Found in the respiratory, digestive, and urinary tracts •MALT (mucosa-associated lymphoid tissue) •Collection of lymphoid tissues linked with the digestive system Lymphoid organs •Lymph nodes – function in the purification of lymph •Afferent lymphatics – carry lymph to nodes •Efferent lymphatics – carry lymph from nodes •Deep cortex dominated by T cells •Outer cortex and medulla contains B cells The Lymphatic System and Immunity The Thymus •Located behind sternum in anterior mediastinum •Capsule •Two lobes •Divided into lobules, each with a cortex and medulla •Cortical lymphocytes surrounded by reticular endothelial cells •Maintain blood–thymus barrier •Secretes thymic hormones: thymosins, thymopoietins, and thymulin The Spleen •Largest mass of lymphoid tissue •Cellular components form pulp •Red pulp contains RBC •White pulp similar to lymphoid nodules •Spleen functions include •Removal of abnormal blood cells and other blood components •Storage of iron •Initiation of the specific immune response Lymphatic system and body defenses •Nonspecific defenses •Do not distinguish one type of threat from another •7 types •Specific defenses •Protect against particular threats •Depend upon the activation of lymphocytes Nonspecific Defenses Nonspecific Defenses, Physical barriers •Keep hazardous organisms outside the body •Includes hair, epithelia, secretions of integumentary and digestive systems (Part 1 - Physical Barriers) Nonspecific Defenses, Phagocytes •Remove cellular debris and respond to invasion by foreign pathogens •Monocyte-macrophage system - Fixed and free •Microphages – Neutrophils and eosinophils •Move by diapedesis •Exhibit chemotaxis (Part 2 - Phagocytes) Nonspecific Defenses, Immunological surveillance •Constant monitoring of normal tissue by NK cells •NK cells •Recognize cell surface markers on foreign cells •Destroy cells with foreign antigens NK cell activation •Recognition of unusual surface proteins •Rotation of the Golgi toward the target cell and production of perforins •Release of perforins by exocytosis •Interaction of perforins causing cell lysis (Part 3 - Immunological Surveillance) Nonspecific Defenses, Interferons (cytokines) •Small proteins released by virally infected cells •Trigger the production of antiviral proteins •Three major types of interferons are: •Alpha– produced by leukocytes and attract/stimulate NK cells •Beta– secreted by fibroblasts causing slow inflammation •Gamma – secreted by T cells and NK cells stimulate macrophage activity (Part 4 - Interferons) Nonspecific Defenses, Complement system •Cascade of ~11 plasma complement proteins (C) •Destroy target cell membranes •Stimulate inflammation •Attract phagocytes •Enhance phagocytosis Complement proteins interact with on another via two pathways •Classical •Alternative (Part 5 - Complement System) Nonspecific Defenses, Inflammation •Localized tissue response to injury producing •Swelling •Redness •Heat •Pain •Effects of inflammation include •Temporary repair of injury •Slowing the spread of pathogens •Mobilization of local, regional, and systemic defenses (Part 6 - Inflammatory Response) Nonspecific Defenses, Fever •Maintenance of a body temperature above 37.2 C (99 F)o •Pyrogens reset the hypothalamic thermostat and raise body temperature •Pathogens, toxins, antigen-antibody complexes can act as pyrogens (Part 7 - Fever) Specific Defenses Forms of immunity •Innate i
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