LYMPHATIC SYSTEM

• The lymphatic system consists of two semi-independent parts: lymphatic vessels and lymphoid tissues and organs
• The lymphatic system functions to transport escaped fluids from the cardiovascular system back to the blood and plays essential roles in body defense and resistance to disease
• If fluids are not picked up by lymphatic vessels

  Edema occurs as fluid accumulates in tissues
• Lymphatic vessels form a one-way system where lymph flows only toward the heart
• Lymph capillaries


  • Weave between tissue cells and blood capillaries
  • Walls overlap to form flaplike minivalves
  • Fluid leaks into lymph capillaries
  • Capillaries are anchored to connective tissue by filaments
  • Higher pressure on the inside closes minivalves, forcing fluid along the vessel
• The right lymphatic duct drains the lymph from the right arm and the right side of the head and thorax, while the thoracic duct drains lymph from the rest of the body
• Lymphatic vessels


  • They are similar to veins of the cardiovascular system - thin-walled, larger vessels have valves, and they are a low-pressure, pumpless system
  • Lymph transport is aided by the milking action of skeletal muscles, pressure changes in the thorax during breathing, and smooth muscle in the walls of lymphatics
• Lymph nodes filter lymph before it is returned to the blood, removing harmful materials like bacteria, viruses, cancer cells, and cell debris
• Lymph nodes


  • Defense cells within lymph nodes include macrophages that engulf and destroy bacteria, viruses, and other foreign substances, and lymphocytes that respond to foreign substances in lymph
  • Most lymph nodes are kidney-shaped, less than 1 inch long, and buried in connective tissue
  • They are surrounded by a capsule, divided into compartments by trabeculae, with a cortex containing follicles and a medulla containing phagocytic macrophages
  • Lymph enters the convex side through afferent lymphatic vessels, flows through sinuses inside the node, and exits through fewer efferent lymphatic vessels, slowing the flow
• Several other lymphoid organs contribute to lymphatic function, including the spleen, thymus, tonsils, Peyer's patches, and appendix
• Spleen


  Located on the left side of the abdomen, it filters and cleans blood of bacteria, viruses, and debris, provides a site for lymphocyte proliferation and immune surveillance, destroys worn-out blood cells, forms blood cells in the fetus, and acts as a blood reservoir
• Thymus


  Found overlying the heart, it functions at peak levels only during youth
• Tonsils


  Small masses of lymphoid tissue deep to the mucosa surrounding the pharynx (throat), trapping and removing bacteria and other foreign pathogens
• Peyer's Patches


  Found in the wall of the small intestine, similar lymphoid follicles are found in the appendix, where macrophages capture and destroy bacteria in the intestine
• Mucosa-associated Lymphoid tissue (MALT)


  Includes Peyer's patches, tonsils, and appendix, acting as a sentinel to protect respiratory and digestive tracts
• The immune system has two mechanisms that defend the body from foreign materials: the innate (nonspecific) defense system and the adaptive (specific) defense system
• Immunity is the specific resistance to disease
• The immune system is a functional system rather than an anatomical organ system
• Innate (nonspecific) defense system


  • Mechanisms protect against a variety of invaders and respond immediately to protect the body from foreign materials
• Adaptive (specific) defense system


  • Fights invaders that get past the innate system, requiring specific defense for each type of invader
• Innate body defenses are mechanical barriers to pathogens and include body surface coverings, specialized human cells, and chemicals produced by the body
• Surface membrane barriers (skin and mucous membranes)

  Provide the first line of defense against the invasion of microorganisms, with protective secretions that inhibit bacterial growth, trap microorganisms, and kill pathogens
• Cells and chemicals providing a second line of defense


  • Include natural killer cells, phagocytes, the inflammatory response, and chemicals that kill pathogens and cause fever
• Natural killer (NK) cells


  Lyse and kill cancer cells and virus-infected cells, releasing chemicals to degrade target cell contents
• Inflammatory response


  Triggered when body tissues are injured, with four cardinal signs (redness, heat, pain, swelling) caused by the release of inflammatory chemicals that dilate blood vessels, make capillaries leaky, and attract phagocytes and white blood cells to the area, preventing spread of damaging agents, disposing of cell debris and pathogens, and setting the stage for repair
• Cytotoxic response


  Chemicals that kill pathogens
• Fever

  Natural killer (NK) cells lyse (burst) and kill cancer cells, virus-infected cells
• Inflammatory response


  1. Triggered when body tissues are injured

  2. Four most common indicators (cardinal signs) of acute inflammation: redness, heat, pain, swelling (edema)
• Inflammatory response (continued)


  1. Damaged cells release inflammatory chemicals (histamine, kinin)

  2. Blood vessels dilate

  3. Capillaries become leaky

  4. Phagocytes and white blood cells move into the area (positive chemotaxis)
• Functions of the inflammatory response

  • Prevents spread of damaging agents
  • Disposes of cell debris and pathogens through phagocytosis
  • Sets the stage for repair
• Process of the inflammatory response

  1. Neutrophils migrate to the area of inflammation by rolling along the vessel wall

  2. Neutrophils squeeze through the capillary walls by diapedesis to sites of inflammation

  3. Neutrophils gather in the precise site of tissue injury (positive chemotaxis) and consume any foreign material present
• Phagocytes


  • Cells such as neutrophils and macrophages engulf foreign material by phagocytosis
  • The phagocytic vesicle is fused with a lysosome, and enzymes digest the cell's contents
• Antimicrobial proteins


  • Enhance innate defenses by attacking microorganisms directly and hindering reproduction of microorganisms
  • Most important types: Complement proteins, Interferon
• Complement proteins


  • Complement refers to a group of at least 20 plasma proteins that circulate in the plasma
  • Complement is activated when these plasma proteins encounter and attach to cells (known as complement fixation)
  • Membrane attack complexes (MACs), one result of complement fixation, produce holes or pores in cells
  • Pores allow water to rush into the cell, causing the cell to burst (lyse)
  • Activated complement enhances the inflammatory response
• Interferons


  • Small proteins secreted by virus-infected cells
  • Interferons bind to membrane receptors on healthy cell surfaces to interfere with the ability of viruses to multiply
• Fever


  • Abnormally high body temperature is a systemic response to invasion by microorganisms
  • Hypothalamus regulates body temperature at 37ºC (98.6ºF)
  • The hypothalamus thermostat can be reset higher by pyrogens (secreted by white blood cells)
  • High temperatures inhibit the release of iron and zinc (needed by bacteria) from the liver and spleen
  • Fever also increases the speed of repair processes
• Adaptive (Specific) Body Defense: THIRD LINE


  • Adaptive body defenses are the body's specific defense system
  • Immune response is the immune system's response to a threat
  • Antigens are targeted and destroyed by antibodies
  • Three aspects of adaptive defense: Antigen specific, Systemic, Memory
  • Two arms of the adaptive defense system: Humoral immunity (antibody-mediated), Cellular immunity (cell-mediated)
• Antigens


  • Antigens are any substance capable of exciting the immune system and provoking an immune response
  • Examples of common nonself antigens: Foreign proteins, Nucleic acids, Large carbohydrates, Some lipids, Pollen grains, Microorganisms (bacteria, fungi, viruses)
  • Self-antigens are human cells' protein and carbohydrate molecules that do not trigger an immune response
  • Haptens are incomplete antigens that can link up with our own proteins and be recognized as foreign
• Cells of the adaptive defense system

  • Lymphocytes (B cells and T cells) respond to specific antigens
  • Antigen-presenting cells (APCs) help the lymphocytes but do not respond to specific antigens
• Lymphocyte differentiation


  1. Lymphocytes arise from hemocytoblasts of bone marrow

  2. Whether a lymphocyte matures into a B cell or T cell depends on where it becomes immunocompetent

  3. Immunocompetence is the capability to respond to a specific antigen

  4. T cells develop immunocompetence in the thymus and oversee cell-mediated immunity

  5. B cells develop immunocompetence in bone marrow and provide humoral immunity

  6. Immunocompetent T and B lymphocytes migrate to the lymph nodes and spleen, where encounters with antigens occur

  7. Differentiation from naïve cells into mature lymphocytes is complete when they bind with recognized antigens