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Anatomy and Physiology. Chapter 14. Introduction The lymphatic system is comprised of a network of vessels, cells, organs, and glands that produce and transport body fluids. Lymphatic vessels collect and carry away excess fluid from interstitial spaces

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Anatomy

and

Physiology

Chapter 14


  • Introduction

  • The lymphatic system is comprised of a network of vessels, cells, organs, and glands that produce and transport body fluids.

  • Lymphatic vessels collect and carry away excess fluid from interstitial spaces

  • The organs of the lymphatic system help defend against disease.

  • Lymphatic capillaries are tiny, closed-ended tubes that extend into interstitial spaces.

    • They receive fluid through their thin walls and once inside, tissue fluid is called lymph.

    • The walls of lymphatic vessels are like veins but with thinner walls.

  • Larger lymphatic vessels pass through lymph nodes and merge to form lymphatic trunks.


  • Lymphatic Trunks and Collecting Ducts

    • The lymphatic trunks drain lymph from the body and are named for the regions they drain.

    • These trunks join one of two collecting ducts - either the thoracic duct or right lymphatic duct.

    • The fluid is eventually returned to the subclavian vein.


  • Lymph Fluid

    • Lymph is made up of water and dissolved substances that leave blood capillaries by filtration and diffusion.

      • Most of the small molecules are returned to the capillaries by diffusion.

    • Lymph then transports small foreign particles (bacteria, viruses, etc.) to lymph nodes.

    • Forces that move blood in veins are the forces that propel lymph through lymphatic vessels.


  • Lymph Nodes

  • Lymph nodes are located along lymphatic pathways.

  • They are bean-shaped, with two important parts:

    • The hilum – area where blood vessels and nerves join a node

    • The medulla – inner area where macrophages and T-cells are.

  • They are covered with connective tissue that extends inside the node and divides it into nodules and spaces called sinuses.

  • These contain both lymphocytes and macrophages which clean the lymph as it flows through the node.

    • Lymph nodes are centers of lymphocyte production, which function in immune surveillance.

    • The macrophages and lymphocytes within lymph nodes filter lymph and remove bacteria and cellular debris before lymph is returned to the blood.

  • The lymph nodes generally occur in chains along the parts of the larger lymphatic vessels.


  • Thymus

  • The thymus is a soft, bi-lobed organ located behind the sternum

  • It shrinks in size during the lifetime (large in children, microscopic in the elderly).

  • The thymus gland is divided into lobules, which are small clusters of thymus cells.

  • Lobules contain lymphocytes, some of which mature into T lymphocytes (T cells) that leave the thymus to provide immunity.

  • The thymus secretes the hormone thymosin, which influences the maturation of T lymphocytes once they leave the thymus.


  • Spleen

  • The spleen lies in the upper left abdominal cavity and is the body’s largest lymphatic organ.

  • The spleen resembles a large lymph node except that it contains blood instead of lymph.

  • Inside the spleen lies white pulp (containing many lymphocytes) and red pulp (containing red blood cells, macrophages, and lymphocytes).

  • The spleen filters the blood and removes damaged blood cells and bacteria.


  • Body Defenses Against Infection

  • Disease-causing agents, also called pathogens, can produce infections within the body.

  • The body has two lines of defense against pathogens:

    • Nonspecific defenses that guard against any pathogen

    • Specific defenses (immunity) that mount a response against a very specific target.

      • Specific defenses are carried out by lymphocytes that recognize a specific invader.

    • Nonspecific and specific defenses work together to protect the body against infection.


  • Innate (Nonspecific) Defenses

  • Species Resistance

    • A species is resistant to diseases that affect other species because it has a unique environment.

      • Pathogens from dogs tend not to infect humans.

      • Doesn’t always work – H5N1 (Bird Flu)

  • Mechanical Barriers

    • The unbroken skin and mucous membranes of the body create mechanical barriers that prevent the entry of certain pathogens.

      • Mechanical barriers represent the body’s first line of defense.

  • Chemical Barriers

    • Chemical barriers, such as the highly acidic and caustic environment of the stomach.

      • Interferons are produced by cells when they are infected with viruses and induce nearby cells to produce antiviral enzymes that protect them from infection.


  • Fever

    • Fever offers powerful protection against infection by interfering with the proper conditions that promote bacterial growth.

    • During fever, the amount of iron in the blood is reduced

      • Fewer nutrients are available to support the growth of pathogens.

    • Phagocytic cells attack with greater vigor when the temperature rises.

  • Inflammation

    • Inflammation, a tissue response to a pathogen, is characterized by redness, swelling, heat, and pain.

    • Major actions that occur during an inflammatory response include:

      • Dilation of blood vessels

      • Increase of blood volume in affected areas

      • Invasion of white blood cells into the affected area

      • Appearance of fibroblasts and their production of a sac around the area.


  • Adaptive (Specific) Defenses or Immunity

    • The response mounted by the body against specific, recognized foreign molecules.

  • Antigens

    • Before birth, the body makes an inventory of "self" proteins and other large molecules.

    • Antigens are generally larger molecules that elicit an immune response.

  • Lymphocyte Origins

    • During fetal development, red bone marrow releases lymphocytes into circulation

      • 70-80% become T lymphocytes (T cells)

      • The remainder become B lymphocytes (B cells).

    • Undifferentiated lymphocytes that reach the thymus become T cells and B cells are thought to mature in the bone marrow.

    • Both B and T cells reside in lymphatic organs.


  • Lymphocyte Functions

    • T cells attack foreign, antigen-bearing cells, such as bacteria, by direct cell-to-cell contact, providing cell-mediated immunity.

    • T cells also secrete cytokines that enhance cellular response to antigens.

    • T cells may also secrete toxins that kill target cells

    • They can also produce growth-inhibiting factors or interferon to interfere with viruses and tumor cells.

    • B cells attack pathogens by differentiating into plasma cells that secrete antibodies (immunoglobulins).


  • There are three main types of T-Cells.

  • Helper T-Cells are cells that help activate B-cells to produce antibodies.

    • They must come in contact with a cell that has already encountered the antigen.

    • Macrophages contain Major Histocompatibility Complex (MHC) proteins that act as both ID badges AND antigen presenting proteins.

    • If a Helper T-Cell comes in contact with a macrophage which is presenting an antigen, it becomes activated.

    • Once activated, the Helper T-Cell then tells other B-cells to produce antibodies to the antigen.

  • Cytotoxic T cells monitor the body's cells, recognizing and eliminating tumor cells and virus-infected cells.

    • Cytotoxic T cells become activated when a antigen binds to its receptors.

  • Memory T cells provide a no-delay response to any future exposure to the same antigen.


  • B Cells and the Humoral Immune Response

    • Most B cells need helper T cells for activation.

      • The helper T cell releases cytokines that activate the B cell so that it can divide and form a clone.

    • B cells may become activated and produce a clone of cells when its antigen receptor encounters its matching antigen.

    • Some of the B cells become plasma cells, producing and secreting antibodies.

    • Like T cells, some of the B cells become memory cells to respond to future encounters with the antigen.


  • Types of Antibodies

  • There are five major types of antibodies (immunoglobulins) that constitute the gamma globulin fraction of the plasma.

    • IgG is in tissue fluid and plasma and defends against bacterial cells, viruses, and toxins and activates complement.

    • IgA is in exocrine gland secretions (breast milk, saliva, tears) and defends against bacteria and viruses.

    • IgM is found in plasma and activates complement and reacts with blood cells during transfusions.

    • IgD is found on the surface of most B lymphocytes and functions in B cell activation.

    • IgE is found in exocrine gland secretions and promotes allergic reactions


  • Antibody Actions

  • Antibodies can react to antigens in three ways:

    • Direct attack

      • Direct attack methods include agglutination, precipitation, and neutralization of antigens.

    • Activation of complement

      • This can produce inflammation or lysis in target cells or antigens.

    • Stimulation of changes in areas that help prevent the spread of the pathogens.


  • Immune Responses

  • When B or T cells become activated the first time, their actions constitute a primary immune response

    • This is when the cells acquire information about the pathogen and the antigens it contains.

    • After this, some cells remain as memory cells.

  • If the same antigen is encountered again, more numerous memory cells can mount a more rapid response, known as the secondary immune response.

    • The ability to produce a secondary immune response may be long-lasting.

  • Immunity is gained in an Active or Passive manner.

    • Active immunity involves actual infection with pathogen.

      • Allows body to produce its own antibodies.

    • Passive immunity involves injection of actual antibodies.

      • Short lived


  • Allergic Reactions

    • Allergic reactions to allergens are excessive immune responses that may lead to tissue damage.

    • Delayed-reaction allergy results from repeated exposure to substances that cause inflammatory reactions in the skin.

    • Immediate-reaction allergy is an inherited ability to overproduce IgE.

    • During allergic reactions, mast cells release histamine and leukotrienes, producing a variety of effects.

    • Allergy mediators sometimes flood the body, resulting in anaphylactic shock, a severe form of immediate reaction allergy.


  • HIV

  • HIV virus infects macrophages through special receptors on the cell surface

  • Once virus is inside cell, it replicates and produces thousands of copies of itself.

  • It then begins infecting the Helper T-Cells and they begin to die at a rapid rate.

    • This affects B-Cell activation and thus antibody production.

  • Eventually HIV begins binding to cytotoxic T-cells

    • This lowers the body’s ability to patrol itself and prevent bacterial infections and cancers.



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