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     Chapter 14
   The lymphatic system is part of the
    circulatory system.
   Two key roles in the body:
       Carries fluid from the extracellular
        environment into the bloodstream, and
        thereby helps to maintain fluid balance.
       It performs the primary role in defending the
        body against disease.
Specialized organs of Lymphatic

Lymph  nodes, the
spleen, the thymus, the
tonsils, and Peyer’s
         The Lymphatic Network
   Interstitial fluid is formed when plasma
    from the bloodstream is pushed through
    capillary walls.
   This watery fluid serves as a transport
    medium for gases, nutrients, and other
    molecules that travel between cells and
    the blood.
   Most of the interstitial fluid (about 90%) diffuses
    back into capillaries for the return trip to the
    heart, but some of it does not.
   How is the remaining interstitial fluid returned?
       The accumulation of interestitial fluid in the extra
        cellular space generates a pressure gradient, which
        creates enough force to literally push the remaining
        interestitial fluid into the lymphatic network.
   This series of veinlike tubes drains
    interstitial fluid and returns it to the
    bloodstream in a one-way flow that moves
    slowly toward the heart.
   Once the interstitial fluid enters the
    lymphatic network, it is referred to as
          Lymphatic Capillaries
   Tiny vessels are found between cells and
    among capillary beds of nearly all tissues
    and organs in the body.
   Areas where they are not found include
    the brain and spinal cord, bone tissue, and
             Lymphatic Vessels
   Their structure resembles veins, as their walls
    consists of the same three layers of tissue with
    the inner endothelium forming one-way valves.
   The valves of lymphatic vessels help prevent the
    backflow of lymph.
   As lymphatic vessels extend toward the heart,
    they gradually increase in size and merge to
    form the larger lymphatic trunks and collecting
   Along their course they lead to small, oval
    organs called lymph nodes, and exit from
    them to continue on their way.
Lymphatic Trunks and Collecting
   Lymphatic trunks are formed from the
    merging of numerous lymphatic vessels.
   They extend a short distance toward the
    heart until they converge to form one of
    the two collecting ducts in the lymphatic
    network: the thoracic duct or the right
    lymphatic duct.
   The thoracic duct is the main collecting vessel
    for the lymphatic network.
   It drains lymph from the left side of the head,
    neck, and thorax; the left upper limb; and the
    entire body below the diaphragm.
   It originates in the lower abdomen as an
    expanded vessel (cysterna chyli) and travels
    upward against the back wall to the upper chest.
   The thoracic duct empties lymph into the
    bloodstream where it unites with the left
    subclavian vein.
   The right lymphatic duct drains lymph from the
    right side of the head, neck, and thorax and the
    right upper limb.
   It extends a short distance in the neck to empty
    lymph into the right subclavian vein, with which it
            Movement of lymph
   Lymph is propelled through the lymphatic
    network by pressure gradients.
   Gradients are established by a number of
    factors, including the accumulation of
    protein in the interstitial fluid, skeletal
    muscle contraction, and breathing
               Lymph Formation
   Interstitial fluid is formed by the movement of
    blood plasma out of blood capillaries.
   It is composed of water and dissolved
    substances, but it lacks larger substances such
    as cells and large proteins.
   The concentration of small proteins in the
    remaining interstitial fluid tends to rise, causing
    the osmotic pressure of the fluid to rise as well.
   Recall that osmotic pressure rises when
    the solute concentrations rise; in this case,
    the proteins are the solutes.
               Flow of lymph
   As lymph begins to accumulate in the
    blind-ended capillaries, the pressure in
    these tiny tubes rises.
   This provides the force to move the lymph
    into the larger lymphatic vessels, where
    the pressure is lower.
         Other Lymphatic Organs
   Lymphatic organs other than the vessels consist
    mostly of packed lymphocytes that perform
    defensive functions.
   Lymphoid Tissue contains collections of
    lymphocytes, a type of white blood cell.
   Lymphocytes play a role in mounting a
    defensive immune response to foreign materials
    such as pathogenic microorganisms.
                Lymph Nodes
   Lymph nodes are small, oval masses of
    lymphoid tissue located along the
    lymphatic pathways.
   There are hundreds of nodes in the body:
    nodes are usually found in clusters or
    chains in the neck, armpit, groin, and deep
    within the abdominal cavity.
         Lymph Node Structure
   Most are shaped like a kidney bean and
    are less than 2.5 cm in length.
   A lymph node is a receiving and sending
    station for lymph.
   Along its convex margin it receives
    incoming lymph by way of lymphatic
    vessels that merge with it.
   These vessels are called afferent
    lymphatic vessels.
   At its concave margin, known as the
    hilus, the node sends lymph on its way
    toward the heart by way of efferent
    lymphatic vessels.
   The internal structure consists of two
       An outer area, called the cortex, and an inner
        part, known as the medulla.
       The cortex consists of many lymph nodules,
        where lymphocytes and macrophages are
       It is also the site of lymphocyte maturation.
   the medulla, the lymphocytes
 In
 are arranged in strands called
 medullary cords.
          Lymph Node Function
   Microorganisms and other foreign particles
    carried by the lymph are filtered and
    removed. (bacteria, toxins released by
    bacteria, and viruses)
   These are inactivated by large numbers of
    lymphocytes that are packed within the
    lymph nodes.
 Macrophages    are also
 present; they engulf damaged
 cells and cell debris.
   Largest of the lymphatic organs.
   It is enveloped by a fibrous capsule whose
    protein fibers divide it internally into
    compartments, many of which contain
    lymphocytes and macrophages.
   The spleen also contains venous sinuses
    that are normally filled with large numbers of
    red blood cells.
   These areas are referred to as red pulp.
The regions of the spleen that contain white blood
  cells are called the white pulp.
The spleen functions as a large filter for
  removing foreign particles, old and defective
  red blood cells, and platelets from the
These functions are accomplished by the
  lymphocytes and macrophages Within the
  white pulp.
   The spleen also provides a blood
    reservoir, as it stores a large volume
    of blood that can be called upon
    during emergency situations involving
    blood loss.
                Thymus Gland
   The Thymus Gland also plays a role in the
    Endocrine System.
   In infants, it is a large, bilobed gland
    located in the thoracic cavity, where it lies
    behind the sternum.
   In adults it is very much reduced in size
    and function.
   During infancy, the thymus cells promote the
    maturity of lymphocytes into T lymphocytes (T
    cells) and releases them into the bloodstream.
   T cells play an important role in the immune
   In adolescence, the thymus gland begins to
    atrophy, and by adult age it has been replaced
    by fibrous and fatty tissue.
   Located in the mouth and throat, where
    they lie partially embedded within the
    mucous membrane.
   They are named according to their
       Palatine tonsils—on the back end of the
        palate (2)
       Pharyngeal tonsils—lie against the wall of
        the upper throat (2)
   Lingual tonsils---at the base of the tongue
   These cells gather and engulf disease-
    causing microorganisms in the mouth and
    throat regions.
   During an infection, the palatine tonsils in
    particular may enlarge because of the
    proliferation of bacteria and white blood cells.
   Consequently, you have a sore throat!
 Ifyou have frequent infections,
  they may remove your tonsils by
  way of a procedure called
              Peyer’s Patches
   Isolated clusters of lymphoid tissue in the
    wall of the small intestine near its distal
   They contain lymphocytes and
    macrophages in large numbers.
   They identify microorganisms and other
    foreign particles that reside in the
 The macrophages provide a
 defensive barrier, preventing the
 movement of bacteria across the
 intestinal wall and into the
 abdominal cavity.
The Defense Mechanisms of the
   All self cells contain a similar distribution
    of membrane receptors which can be read
    by white blood cells as “code” and which
    differs from the distribution of membrane
    receptors found in nonself cells.
   The specific distribution of membrane
    receptors is determined by a group of
    genes in the chromosomes of all self cells,
    called major histocompatibility complex
   Two general types of defensive strategies that
    the body employs:
      nonspecific mechanisms: nonspecific
       defense mechanisms help prevent the
       entrance of foreign material into the body.
      An example is the unselective destruction of
       all bacteria by phagocytosis.
   Specific defense mechanisms: this keys
    into the particular type of pathogen or
    toxin that is infecting the body.
   Once the invaders have been identified,
    they may be inactivated by a variety of
    defensive mechanisms.
   The selective production of special
    proteins, called antibodies, is one way of
    inactivating the invaders.
 The specific defense mechanisms
 are collectively referred to as the
 immune response.
     Nonspecific Mechanisms

 There are many defense
  mechanisms that are
 The most important include
  physical barriers, Phagocytosis,
  natural killer cells, proteins, and
             Physical Barriers
   Most important barrier is the skin.
   Mucous membranes provide a second
    important physical barrier.
   Mucous membranes also produce
    chemicals that help in the defensive
   An invasion by a pathogen that
    successfully penetrates the body’s
    physical barriers is called an infection.
   Phagocytosis is the ingestion and
    destruction of particles by specialized
   Once collected, the particle is digested by
    vacuoles inside the cell.
   A cell that performs this function is called
    a phagocyte.
   Phagocytosis is a type of nonspecific
    defense because the white blood cells
    performing this function do not
    distinguish between different types of
    foreign cells.
   Phagocytosis is performed by most
    white blood cells.
   The most active in this defensive activity are
    monocytes and neutrophils, which squeeze
    through blood vessel walls to reach sites of
    infection and inflammation.
   Monocytes give rise to their more mature form,
    known as macrophages, which often become
    attached to the walls of lymphatic vessels and
    other lymphatic organs.
   Macrophages may also wander freely
    throughout the circulatory system, or they
    may fix themselves to the walls of blood
    vessels, the liver, and the lungs.
   Attached macrophages constitute an
    important part of the body’s defense
    network, known as mononuclear
    phagocytic system.
             Natural Killer Cells
   Natural killer cells are a type of white
    blood cell that kills invading foreign cells
    nonspecifically by a method other than
   Natural killer cells are unique in that they
    kill cancer cells and virus-infected cells by
    puncturing a hole in their plasma
    membrane (called cell lysis).
 Itis currently thought that
  natural killer cells are a type
  of lymphocyte.
   Certain groups of proteins play a role in
    nonspecific defense.
   One group called complement; it includes
    more than 20 types of plasma proteins
    that are normally in an inactive state.
   COMPLEMENT becomes activated by the
    onset of an infection.
   Once activated, it labels the foreign
    substance as unwanted, enabling their
    identification by phagocytes.
   A second group of proteins in nonspecific
    defense are the interferons.
   They are the body’s main defense
    against viruses.
   Interferons are secreted by cells that have
    become infected by viruses.
   They diffuse to nearby cells and bind to
    their membranes. This causes an
    interference in the ability of viruses to
    proliferate in these cells.
   Interferon therapy is used in the treatment
    of hairy-cell leukemia.
   Inflammation is a response to body stress,
    or disruption of homeostasis, which often
    follows an infection or physical injury.
   It is a nonspecific mechanism because it
    may occur in any tissue of the body.
   The inflammatory responses are initiated
    when a damaged cell releases substances
    into the bloodstream.
 These substances include histamine
  and serotonin.
 They produce two main responses by
  local tissue:
    Vasodilation of blood vessels

    Increased permeability of blood
 These responses produce four
  Redness

  Swelling

  Heat

  Pain
 The leaking of fluids into the
 extracellular space is also
 known as EDEMA. (swelling)
    Specific mechanisms: The
       Immune Response
 The  immune response relies
  upon the ability of lymphocytes
  to recognize specific antigens.
 The defense mechanisms that
  are specific constitute the
  immune response of the body,
  or immunity.
   Two types of immune responses:
     Cell-mediated immunity—cells
      provide the main defensive
     Humoral immunity---utilizes
      antibodies as the main weapon
      against invaders.
        Components of Immunity
   ANTIGENS—substances that provoke an
    immune response when they enter the
   They are recognized by white blood cells
    as foreign.
   Antigens include toxic molecules released
    by microorganisms, or the whole
    microorganisms themselves.
   An antibody is a protein molecule that is
    produced in response to a specific
   It binds to the antigen to form an antigen-
    antibody complex, which inactivates the
    toxic effects of the antigen.
   Antibodies belong to a family of proteins
    known as immunoglobulins (Ig).
   There are five classes of Ig:
       IgG--- found in the blood, lymph, intestines
       IgA---in tears, salvia, mucus, milk, blood, and
       IgM and IgD---in blood and lymph, and
        attached to surfaces of B cells
       IgE---attached to surfaces of basophil cells,
        mast cells, and B cells.
   The most important type of white blood cell in
    immunity is the lymphocyte.
   Lymphocytes originate from the red bone
    marrow before birth.
   Soon after their production, about half of them
    migrate to the thymus gland.
   Once within the tissues of the thymus gland,
    these cells mature into a form of lymphocyte
    called T cells.
   The rest migrate to the spleen or Peyer’s
    patches, where they mature into B cells.
   Soon after this maturation process, T cells
    and B cells leave their site of development
    and migrate to lymphoid tissue.
   Here they become “programmed” to
    recognize self cells and distinguish them
    from nonself cells.
   Once a lymphocyte has developed this
    ability, it is called immunocompetent.
   A T cell that has become sensitized
    develops special properties that enable it
    to react against the particular antigen.
   These properties are passed on to its
    descendent cells, producing a cell line with
    identical properties, they are known as
   Killer T cells are specialized in destroying
    virus-infected cells, cancer cells, and
    foreign cells.
   They bind to these cells once they are
    recognized and release toxic chemicals
    called lymphotoxins.
   Killer T cells are also called cytolytic T
   Helper T cells---when they identify an antigen,
    they release proteins into the bloodstream that
    signal other cells to attack the invader.
   These proteins, called lymphokines, stimulate
    killer T cells to grow and divide, attract
    neutrophils and monocytes to the site of
    intrusion, and enhance the ability of
    macrophages to ingest and destroy nonself
 Thehelper T cells orchestrate the
 defensive symphony of the body.
   Memory T cells----provide the immune response
    with a “memory” of the specific antigen that
    caused the initial sensitization.
   B Cells---account for about 20% of the
    lymphocyte population in a normal blood
   In B cells, most of the descendents of the
    sensitized cell become plasma cells.
   Plasma cells swing into action to synthesize vast
    amounts of antibodies at an incredible rate
    (2000 antibody molecules per second).
   The plasma cells are the only cells of the body
    that produce antibodies.
   Memory B cells---found mainly in the bone
    marrow. They provide a “memory bank” of
    sensitized cells that can respond to the same
    antigen when it is encountered again.
          Cell-Mediated Immunity
   Provided by the various types of T cells.
   Initiated when a macrophage identifies an
    antigen, phagocytizes it, and processes it.
   Processed antigen is expressed on the
    macrophage plasma membrane.
   Macrophage then presents the antigen to a T
    cell, resulting in T cell sensitization.
   The sensitized T cell then proliferates into
    various T-cell lines (killer, helper, suppressor,
    and memory).

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