Chapter 22: Lymphatic System and Lymphoid Organs and Tissues

• Lymphatic system: returns fluids that have leaked from the blood vascular system back into the blood

  • consists of 3 parts:
  • a network of lymphatic vessels
  • lymph = fluid contained in those vessels
  • lymph nodes - cleanse the lymph as it passes through them
• Lymphoid organs and tissues: provide the structural basis of the immune system

  • house phagocytic cells and lymphocytes which play roles in the body's defense mechanisms and resistance to disease
  • eg. spleen
• Lymphatic System:

  • as blood circulates through the body there is an exchange of nutrients, wastes and gases between the blood and the interstitial fluid
  • some fluid is not reabsorbed and must be returned to the blood in order for the cardiovascular system to operate properly
  • lymphatic vessels are elaborate networks of drainage vessels that collect the excess interstitial fluid and return it to the bloodstream
  • Lymphatic vessels form a one-way system in which lymph flows only towards the heart
• Lymphatic capillaries:

  • weave between the tissue cells and blood capillaries in the loose connective tissues of the body
  • 2 unique structural modifications allow these vessels to be extremely permeable - more permeable than blood capillaries 
  • endothelial cells forming the wall are not tightly joined, but overlap each other loosely forming easily opened flaplike minivalves
  • collagen filaments anchor the endothelial cells to surrounding structures
  • an increase in interstitial fluid volume will open the minivalves
• Lymphatic capillaries - pressure:

  • fluid pressure in the interstitial space is greater than the pressure in the lymphatic capillary, the minivalve flaps open wide allowing fluid to enter the lymphatic capillary
  • pressure is greater inside the lymphatic capillary, it forces the endothelial minivalve flaps shut preventing lymph from leaking back out as the pressure moves it along the vessel
• Collecting lymphatic vessels:

  • have the same 3 tunics as veins
  • thinner walls and more internal valves
• Lymphatic trunks:

  • drain fairly large areas of the body
  • named mostly for the regions from which they drain lymph
• Lymphatic ducts:

  1. right lymphatic duct - drains lymph from the right upper limb and the right side of the head and thorax
  2. thoracic duct - much larger because receives lymph from the rest of the body
• Lymph flow: lymphatic capillaries (smallest) --> collecting lymphatic vessels --> lymphatic trunks --> lymphatic ducts (largest)
• Lymph transport:

  • lymphatic vessels are low-pressure conduits
  • use the same mechanisms that promote venous return in blood vessels to move the lymph
  • milking action of active skeletal muscles
  • pressure changes in the thorax during breathing
  • valves to prevent backflow
• Innate immunity:

  • external physical and chemical barriers provided by the skin and mucous membranes
  • internal defenses such as antimicrobial substances, natural killer cells, phagocytes, inflammation and fever
• Adaptive immunity:

  • specificity for particular foreign molecules (antigens); distinguishing self from nonself molecules
  • memory for previous encountered antigens
  • what B and T lymphocytes do
• Lymphoid cells:

  • lymphocytes - arise in red bone marrow and mature into one of two main types:
  • T lymphocytes (T cells)
  • B lymphocytes (B cells)
• T lymphocytes (T cells):

  • manage the immune response
  • some directly attack and destroy infected cells
• B lymphocytes (B cells):

  • protect the body by producing plasma cells that secrete antibodies
  • antibodies mark antigens for destruction by phagocytes or other means
• Agranulocytes - Lymphocytes:

  • only about 2% are found in blood; most are found in lymph tissue (lymph nodes, spleen) and mediate immune responses
  • 2 main types of lymphocytes:
  • B-lymphocytes (B cells) - so named because they mature in bone marrow
  • T-lymphocytes (T cells) - so named because they mature in thymus gland (located below thyroid gland in-between lungs)
• B-lymphocytes (B cells) give rise to:

  • plasma cells - produce antibodies that will attach to foreign cells (antigens) and destroy them
  • memory B cells - "remember" pathogens encountered and can have a rapid response if it is detected again
• Clonal selection: process by which a lymphocyte proliferates (divides) and differentiates (form more highly specialized cells) in response to a specific antigen 

  • Forms a population of identical cells (clones) that can recognize the same specific antigen as the original lymphocyte
  • Swollen lymph nodes or tonsils are caused by clonal selection
• T-lymphocytes (T cells) give rise to:

  • effector T cells (cytotoxic T cells) - secrete cytotoxic agents that destroy virus-invaded cells and cancer cells
  • helper T cells - secrete chemical messengers to activate T and B cells as well as macrophages
  • regulatory T cells - act to control immune responses/suppressors
  • memory T cells - "remember" antigen so can have rapid immune response to second encounter
• Marcrophages: play a crucial role in body protection and the immune response

  • phagocytize foreign substances
  • help to activate T cells
• Reticular cells: fibroblast-like cells that produce the reticular fibre stroma

  • becomes the network that supports the other cell types in lymphoid organs and tissues
• Major histocompatibility complex antigens:

  • located in the plasma membrane of nucleated body cells such as white blood cells
  • act as cell identity markers
  • are unique to each individual except identical twins
  • will help recognize antigen is foreign and not self which is an important first step in the adaptive immune response
• Lymphoid tissues - functions:

  1. houses and provides a proliferation site for lymphocytes
  2. is an ideal surveillance vantage point for lymphocytes and macrophages
• Lymphoid tissues - types:

  1. diffuse lymphoid tissue
  2. loose arrangement of lymphoid cells and some reticular fibers
  3. found in virtually every body organ
  4. lymphoid follicles/nodules
  5. solid, spherical bodies consisting of tightly packed lymphoid cells and reticular fibers
  6. follicles form part of larger lymphoid organs such as lymph nodes
• Lymph nodes - function 1: filtration

  • principle lymphoid organs
  • macrophages in the nodes remove and destroy microorganisms/debris that enter the lymph form the connective tissues
  • prevents them from being delivered to the blood and spreading to other parts of the body
• Lymph nodes - function 2: immune system activation

  • sites where lymphocytes will encounter antigens and are activated to mount an attack against them
• Spleen:

  • largest lymphoid organ
  • located in the left side of the abdominal cavity just beneath the diaphragm
  • provides a site for lymphocyte proliferation and immune surveillance and response
  • various blood cleansing functions
  • stores some of the breakdown products of RBC's for later reuse
  • stores blood platelets and monocytes for release into the blood
  • site of erythrocyte production in the fetus
• Histologically the spleen consists of 2 components:

  1. white pulp
  2. where immune functions take place
  3. composed mostly of lymphocytes suspended on reticular fibers
  4. red pulp
  5. where worn-out RBCs and blood borne pathogens are destroyed
  6. contains large numbers of erythrocytes and the macrophages that engulf them
• Thymus:

  • found in the inferior neck region
  • maturation site for T lymphocyte precursors
  • must be kept isolated from foreign antigens to prevent their premature activation
  • there is a blood thymus barrier that keeps blood borne antigens out of the thymus
• Mucosa-Associated lymphoid tissue (MALT): is a set of distributed lymphoid tissues strategically located in mucous membranes throughout the body

  • tonsils
  • Peyer's patches
  • appendix
• Tonsils:

  • ring of lymphoid tissue around the entrance to the pharynx
  • gather and remove many of the pathogens entering the pharynx in food or in inhaled air
  • include palatine, lingual, pharyngeal and tubal tonsils
  • Palatine tonsils: either side at the posterior end of the oral cavity
  • Lingual tonsil: base of tongue
  • Pharyngeal tonsil: posterior wall of nasopharynx
  • Tubal tonsils: surround the openings of the auditory tubes into pharynx
• Peyer's Patches:

  • structurally similar to the tonsils
  • located in the wall of the distal portion of the small intestine (ileum)
• Appendix:

  • tubular offshoot of the first part of the large intestine
  • destroys bacteria
  • generates many "memory" lymphocytes for long-term immunity