Lymphatic System PDF

LYMPHATIC SYSTEM The Lymphatic system Consists of two semi - independent parts ○ A network of lymphatic vessels ○ Lymphoid tissues and organs scattered throughout the body Lymphatic nodules Lymph nodes Tonsils...

LYMPHATIC SYSTEM The Lymphatic system Consists of two semi - independent parts ○ A network of lymphatic vessels ○ Lymphoid tissues and organs scattered throughout the body Lymphatic nodules Lymph nodes Tonsils Spleen Thymus Returns interstitial fluid and leaked plasma proteins back to the blood ○ Lymph: interstitial fluid once it has entered lymphatic vessels  Lymph – “clear water” Provides structural basis of the immune system Play essential role in the body’s defense mechanisms and its resistance to disease FUNCTIONS OF THE LYMPHATIC SYSTEM Fluid Balance ○ Returns three liters of fluid a day back to the circulation Lipid Absorption ○ Lacteals: specialized lymph capillaries present in the intestinal mucosa Absorb digested fat and deliver chyle (fatty lymph) to the blood Defense ○ Defends against microorganisms and foreign substances LYMPHATIC VESSELS Originate as small, dead-end tubes called lymphatic capillaries Excess fluid passes through the tissue spaces and enters the lymphatic capillaries CNS, Bone marrow, cartilage, epidermis and cornea (tissues without blood vessels) lack lymphatic vessels Lack a basement membrane and lined with simple squamous endothelium ○ More permeable than blood capillaries ○ Function as one-way valves that allow fluid to enter but prevent it from passing back LYMPH MOVES THROUGH LYMPHATIC VESSELS THROUGH Contraction of lymphatic vessels Contraction of skeletal muscles Thoracic pressure changes LYMPH NODES Round, oval, or bean-shaped bodies distributed along the various vessels and connected in a series Filter lymph Lymphatic vessels converge into lymphatic trunks then form lymphatic ducts ○ Jugular trunks (head and neck) ○ Subclavian trunks (upper limbs, thoracic wall, mammary glands) ○ Bronchomediastinal trunks (deep thoracic wall and organs) ○ Intestinal trunks ○ Lumbar trunks (lower limbs, pelvic and abdominal wall, kidneys) LYMPHATIC VESSELS Lymph is delivered into one of two large trunks ○ Right lymphatic duct: drains the right upper arm and the right side of the head and thorax ○ Thoracic duct: arises from the cisterna chyli and drains the rest of the body RIGHT LYMPHATIC DUCT Drains lymph from the right upper limb, and the right side of the head and thorax Empties into the blood at the junction of the right internal jugular and subclavian veins THORACIC DUCT Drains lymph from the rest of the body Empties into the blood at the junction of the left internal jugular and subclavian veins. LYMPHATIC TISSUE Lymphatic organs contain lymphatic tissues ○ Consists of lymphocytes, macrophages, dendritic cells, reticular cells and others ○ When the body is exposed to MOs, the lymphocytes divide, increase in number and become part of the immune system ○ Composed of very fine collagen fibers (reticular fibers) from reticular cells where lymphocytes attach and filter the passing fluid Can be encapsulated (most the organs) and nonencapsulated (Mucosa-associated lymphoid tissue – MALT) LYMPHOID TISSUES Diffused Lymphoid tissue - loose arrangement of cells and reticular fibers ○ Found in every body organ ○ Appear in the lamina propria of mucous membranes (GI tract lining) Lymphoid follicles/ nodules – solid, spherical bodies ○ B cells predominate ○ Enlarges when B cells are dividing LYMPHOID ORGANS Primary ○ Thymus ○ Red Bone Marrow Secondary ○ Lymph Nodes ○ Tonsils ○ Spleen ○ Peyer’s patches (aggregated lymphoid nodules in the small intestine) ○ Appendix LYMPHATIC TISSUE AND ORGANS Peyer’s patches: Isolated clusters of lymphoid tissue, similar to tonsils Found in the wall of the distal portion of the small intestine Similar structures are found in the appendix Peyer’s patches and the appendix: ○ Destroy bacteria, preventing them from breaching the intestinal wall ○ Generate “memory” lymphocytes for long-term immunity Tonsils Simplest lymphoid organs Form a ring of lymphatic tissue around the pharynx Location: ○ Palatine tonsils: either side of the posterior end of the oral cavity ○ Lingual tonsils: lie at the base of the tongue ○ Pharyngeal tonsil: posterior wall of the nasopharynx Epithelial tissue overlying tonsil masses invaginates, forming blind-ended crypts Crypts trap and destroy bacteria and particulate matter Lymph Nodes Principal lymphoid organs of the body Embedded in connective tissue and clustered along lymphatic vessels Aggregations of these nodes occur near the body surface in inguinal, axillary, and cervical regions of the body Two basic functions: ○ Filtration: macrophages destroy microorganisms and debris ○ Immune system activation: monitor for antigens and mount an attack against them Nodes are bean shaped and surrounded by a fibrous capsule Trabeculae extended inward from the capsule and divide the node into compartments Nodes have two histologically distinct regions: a cortex and a medulla ○ Cortex contains follicles with germinal centers, heavy with dividing B cells ○ Dendritic cells nearly encapsulate the follicles ○ T cells circulate continuously among the blood, lymph nodes, and lymphatic stream ○ Deep cortex houses T cells in transit Spleen Is in the left superior side of the abdomen Two distinct areas: ○ White pulp: contains mostly lymphocytes suspended on reticular fibers and are involved in immune functions ○ Red pulp: remaining splenic tissue concerned with disposing of worn-out RBCs and bloodborne pathogens The spleen is a limited reservoir for blood Thymus A bilobed organ that secretes hormones (thymosin and thymopoietin) that cause T lymphocytes to become immunocompetent Size of the thymus varies with age: ○ In infants, it is found in the inferior neck and extends into the mediastinum where it partially overlies the heart ○ It increases in size and is most active during childhood ○ It stops growing during adolescence and then gradually atrophies Differs from other lymphoid organs in important ways ○ It functions strictly in T lymphocyte maturation ○ It does not directly fight antigens The stroma of the thymus consists of star-shaped epithelial cells (not reticular fibers) These thymocytes secrete the hormones that stimulate lymphocytes to become immunocompetent IMMUNITY The ability to resist the harmful effects of microorganisms and other foreign substances Adaptive immunity exhibits specificity and memory Innate immunity does not show specificity or memory INNATE IMMUNITY Responds quickly and consists of: ○ Mechanical mechanisms Skin and mucosae prevent entry of microorganisms Tears, saliva, and mucus remove them ○ Chemical mediators ○ Cells ○ Inflammatory Response Innate Immunity Complement Each complement pathway involves a cascade in which compliment proteins are activated in an orderly sequence. The end result is cell lysis, phagocytosis, and inflammation Refers to a group of 20 or so proteins that circulate in the blood in an inactive form Provides a major mechanism for destroying foreign substances in the body Amplifies all aspects of the inflammatory response Kills bacteria and certain other cell types (our cells are immune to complement) Can be activated by either the classical or the alternative pathway ○ Classical pathway is part of adaptive immunity Depends on the binding of antibodies to invading organisms Subsequent binding of C1 to the antigen-antibody complexes (complement fixation) ○ Alternative pathway is part of innate immunity Triggered by interaction among factors B, D, and P, and polysaccharide molecules present on microorganisms Interferons Leave the infected cell and enter neighboring cells Stimulates the neighboring cells to produce proteins to prevent the replication of viruses Activate macrophages and natural killer cells Cells Chemotaxis is the ability of white blood cells to move to tissues that release certain chemicals Phagocytosis is the ingestion and destruction of materials. Neutrophils are small phagocytic cells Basophils and mast cells release chemicals that promote inflammation Eosinophils release enzymes that reduce inflammation Natural killer cells lyse tumor cells and virus-infected cells Innate Immunity Macrophages are large phagocytic cells ○ Can engulf more than neutrophils can ○ In connective tissue they protect the body at locations where microbes are likely to enter, and macrophages clean blood and lymph ○ Some macrophages have specific names Dust cells in the lungs Kupffer cells in the liver Microglia in the CNS Innate immunity Inflammatory Response Can be initiated in many ways ○ Chemical mediators cause vasodilation and increase vascular permeability, which allows the entry of other chemical mediators ○ Chemical mediators attract phagocytes ○ The amount of chemical mediators and phagocytes increases until the cause of the inflammation is destroyed. Then the tissue undergoes repair Local inflammation produces the symptoms of ○ Redness ○ Heat ○ Swelling ○ Pain ○ Loss of function Symptoms of systemic inflammation include ○ An increase in neutrophil numbers ○ Fever ○ Shock ADAPTIVE IMMUNITY The adaptive immune system is a functional system that: ○ Recognizes specific foreign substances ○ Acts to immobilize, neutralize, or destroy foreign substances ○ Amplifies inflammatory response and activates complement The adaptive immune system is antigen-specific, systemic, and has memory It has two separate but overlapping arms: ○ Antibody-mediated immunity (provided by antibodies in the blood and lymph) ○ Cell-mediated immunity (lymphocytes are involved) Antibody is a protein produced by the immune system in response to the presence of an antigen Antigens are large molecules that stimulate an adaptive immune system response Foreign antigens are not produced by the body (self-antigens are) B cells are responsible for antibody-mediated immunity T cells are involved with cell-mediated immunity ORIGIN AND DEVELOPMENT OF LYMPHOCYTES B cells and T cells originate in red bone marrow ○ B cells are processed in bone marrow ○ T cells are processed in the thymus Positive selection ensures the survival of lymphocytes that can read against antigens Negative selection eliminates lymphocytes that react against self-antigens A clone is a group of identical lymphocytes that can respond to a specific antigen. B cells and T cells move to lymphatic tissue from their processing sites  ○ They continually circulate from one lymphatic tissue to another ACTIVATION OF LYMPHOCYTES The antigenic determinant (epitope) is the specific part of the antigen to which the lymphocyte responds The antigen receptor (T-cell receptor or B-cell receptor) on the surface of lymphocytes combines with the antigenic determinant  MHC class I molecules display antigens on the surface of nucleated cells, resulting in the destruction of the cells MHC class II molecules display antigens on the surface of antigen-presenting cells, resulting in the activation of immune cells MHC antigen complex and costimulation are usually necessary to activate lymphocytes ○ Costimulation involves cytokines and certain surface molecules Antigen-presenting cells stimulate the proliferation of helper T cells which stimulate the proliferation of B or T effector cells INHIBITION OF LYMPHOCYTES Tolerance is suppression of the immune system’s response to an antigen Tolerance is produced by The deletion of self-reactive cells The prevention of lymphocyte activation Suppressor T cells ADAPTIVE - ANTIBODY MEDIATED IMMUNITY Antibodies are proteins ○ The variable region of an antibody combines with the antigen ○ The constant region activates complement or binds to cells ○ Five classes of antibodies exist: IgG, 1gM, IgA, IgE, and IgD ADAPTIVE IMMUNITY Antibody - Mediated Immunity Antibodies affect the antigen in many ways ○ Bind to the antigen and interfere with antigen activity or bind the antigens together ○ Increase phagocytosis by binding to the antigen and to macrophages ○ Can activate complement through the classical pathway ○ Attach to mast cells or basophils and cause the release of inflammatory chemicals when the antibody combines with the antigen  Antibody Production ○ The primary response results from the first exposure to an antigen B cells form plasma cells, which produce antibodies and memory B cells ○ The secondary response results from exposure to an antigen after a primary response Memory B cells quickly form plasma cells and additional memory B cells Cell Mediated Immunity Cells infected with intracellular micro-organisms process antigens that combine with MHC class I molecules Cytotoxic T cells are stimulated to divide, producing more cytotoxic T cells and memory T cells, when MHC class I/antigen complexes are presented to T-cell receptors Cytokines released from helper T cells also stimulate cytotoxic T cells Cytotoxic T cells lyse virus-infected cells, tumor cells, and tissue transplants Cytotoxic T cells produce cytokines, which promote phagocytosis and inflammation ORGAN TRANSPLANTS AND PREVENTION OF REJECTION Cell-mediated responses reject grafts and foreign organ transplants unless the recipient is immunosuppressed. Infections are major complications in such patients. INSUFFICIENT OR OVERACTIVE IMMUNE RESPONSES Immunodeficiency diseases include acquired immune deficiency syndrome (AIDS), caused by HIV, and severe combined immunodeficiency (SCID) syndromes. Minor infections become fatally overwhelming because the immune system is unable to combat them. Autoimmune disease occurs when the body regards its own tissues as foreign and mounts an immune attack against them. Examples include rheumatoid arthritis and multiple sclerosis.  Hypersensitivity is an abnormally intense reaction to an otherwise harmless antigen. Immediate hypersensitivities (allergies) are mounted by IgE antibodies. ○ Subacute hypersensitivities, involving both antibodies and complement, include antibody-mediated cytotoxic and immune-complex hypersensitivities. ○ Cell-mediated hypersensitivity is called delayed hypersensitivity.