Lymphatic System

• Immune system


  Consists of 2 semi-independent parts: Lymphatic vessels and Lymphoid tissues & organs

  
• Primary Lymphoid Organs


  • B lymphocytes
  • T lymphocytes
• Secondary Lymphoid Organs


  • Spleen
  • Lymph nodes
  • Mucosal-associated lymphoid tissues (MALT)
• Functions of the immune system

  • Transport fluids back to the blood
  • Play essential roles in body defense and disease resistance
  • Provide defense or immunity against foreign antigens inside the body

  
• ELLIE METCHNIKOFF: 'Russian scientist who discovered and observed phagocytosis (cells that eat cells) under the microscope'
• Antibodies


  Serum proteins produced by certain lymphocytes when exposed to a certain substance

  
• Bone marrow


  • One of the largest tissues in the body and fills the core of all long flat bones
  • Main source of HSC (erythrocytes, granulocytes, monocytes, platelets, and lymphocytes)
• Thymus


  • Small, flat, bilobed organ found in the thorax, or chest cavity, right below the thyroid gland and overlying the heart
  • 30 to 40 g by puberty
  • Produces enough young t lymphocytes early in life
  • Maturation of T cells: 3 weeks
  • Mature T cells released from the medulla
  • Functions at peak levels only during childhood & produces hormones (like thymosin) to program lymphocytes
  • Regulatory T cells form in the thymus upon interacting with dendritic cells presenting self-antigens in a process promoted by cytokines from thymic epithelial cell (TEC) aggregates called Hassall corpuscles, found only in the thymic medulla
• After maturation of T cells, they make their way to secondary lymphoid organs
• Main contact with foreign antigens takes place in secondary lymphoid organs
• Thoracic duct


  Largest lymphatic vessel in the body, collects most of the body's lymph fluid and empties it into the left subclavian vein
• Lymphopoesis


  Making of lymphocytes which occurs in secondary lymphoid organs and is dependent on antigenic stimulation
• Spleen


  • Largest secondary lymphoid organ
  • Located in the upper-left quadrant of the abdomen below the diaphragm
  • Large discriminating filler that removes old and damaged cells and foreign antigens from the blood
  • Receives blood vol. approx. 350 mL/minute and allows lymphocytes and macrophages to constantly survey for infectious agents
• Splenic sinusoids


  Lined by unusual endothelial cells called "stave cells"
• Types of spleen


  • Red pulp
  • White pulp
• Red pulp


  • Filters blood, removes defective erythrocytes, and recycles hemoglobin iron, consists of splenic cords with macrophages and blood cells of all kinds and splenic sinusoids
  • Blood flow in red pulp is either a CLOSE CIRCULATION, moving from capillaries into the venous sinusoids, or OPEN CIRCULATION, capillaries open directly into splenic cords
• White pulp


  • Only 20% of the spleen, is a secondary lymphoid tissue associated with small central arterioles that are also enclosed by PALS (periarteriolar lymphoid sheaths) of T cells
  • Periarteriolar lymphoid sheath (PALS): contains mainly T-cells
  • Primary follicles – contain B cells that are not yet stimulated by antigens
• Lymph nodes


  • Kidney-shaped, <1inch long & covered by a capsule of dense connective tissue, located along lymphatic ducts, numerous near joints, and where the arms and legs join the body
  • Provide the ideal environment for contact with foreign antigens that penetrate the tissues
  • Filtration of interstitial fluid from around cells in the tissues, it allows contact between lymphocytes and foreign antigens from the tissues
  • Filters lymph and provides a site for B-cell activation and differentiation to antibody-secreting plasma cells
• Interstitial fluid


  Fluid that surrounds cells & tissues
• Lymph fluid


  Filtrate of blood and arises from the passage of water and into the interstitial spaces between cells
• Parts and functions of lymphatic vessels

  • Lymph capillaries - Walls overlap to form flap-like mini valves & fluid leaks into them, Higher pressure on the inside closes mini valves
  • Lymphatic collecting vessels/Lymphatic vessels - Collects lymph from lymph capillaries & carries lymph to & away from lymph nodes, Returns fluid to circulatory veins near the heart
• Lymph node structure


  • Sinuses: ideal location for phagocytosis
  • Secondary follicles: consist of antigen-stimulated proliferating B cells
  • Germinal center: transformation of B cells takes place
  • Cortex: outer part, contains follicles (collections of lymphocytes), Paracortex: location of T-cells in lymph nodes
  • Medulla: inner part, contains phagocytic macrophages
• Defense cells in lymph nodes

  • Macrophages: engulf and destroy foreign substances
  • Lymphocytes: provide immune response to antigens

  
• Mucosa-associated lymphatic tissue (MALT)


  • Found in gastrointestinal, respiratory, and urogenital tracts, One of the largest lymphoid organs (70%)
• Examples of MALT


  • Tonsils
  • Appendix
  • Peyer patches (a specialized type of MALT located at lower ileum of the intestinal tract)
• Tonsils


  • Small masses of lymphoid tissue around the pharynx, Trap & remove bacteria & other foreign materials, Palatine tonsils are covered by stratified squamous epithelium with 10-20 deep invaginations or tonsillar crypts, Lingual tonsils are also covered by stratified squamous epithelium with crypts, The single pharyngeal tonsil is situated in the posterior wall of the nasopharynx, is covered by pseudostratified ciliated columnar epithelium, and has a thin underlying capsule
• Peyer's patches


  • Wall of the small intestine, resembling tonsils in structure, Capture & destroy bacteria in the intestine
• Appendix


  • Tubelike offshoot of the 1st part of the large intestine, Has lymphoid follicles, that capture & destroy bacteria
• Parts of the immune system

  • Innate immunity
  • Adaptive immunity

  
• Innate immunity


  The ability to resist infection by means of normally present body functions
• Adaptive immunity


  Acquired gradually by exposure to microorganisms
• Characteristics of innate immunity

  • Non-specific and without memory
  • Mediated by: Granulocytes and other leukocytes
  • Cells involved: Phagocytes, Basophils, Macrophages, Dendritic cells
  • Response time: Immediate
  • Occurrence: Natural
  • Processes involved: Phagocytosis and antigen preservation
  • Specificity: Non-specific
  • Span: Short term
  • Pathogen recognition: toll-like receptors (receptors that allow recognition and binding of surface components of such invaders)

  
• Characteristics of adaptive immunity


  • Specific and with memory
  • Mediated by: lymphocytes and antigen presenting cells (APCs)
  • Cells involved: B cells, T cells, NK cells
  • Response time: Delayed
  • Occurrence: Stimulated
  • Processes involved: T-cell activation & B-cell activation
  • Specificity: Specific
  • Span: Long term
  • Pathogen recognition: memory cells

  
• First line of defense in innate immunity

  • Skin: Forms mechanical barrier to foreign materials, Acid mantle, Keratin
  • Intact mucous membranes: Form mechanical barrier that prevents entry of pathogens, Mucus, Nasal hairs, Cilia, Gastric juice, Acid mantle of vagina, Lacrimal secretion (tears), saliva
• Second line of defense in innate immunity

  • Phagocytes (neutrophils & macrophages)
  • Natural killer (NK) cells: promote cell lysis (releasing perforins) by direct cell attack against virus-infected/cancerous body cells
  • Inflammatory response: Triggered when body tissues are injured, 5 cardinal signs, Results in a chain of events leading to protection and healing

  
• Cells of innate immunity

  • Neutrophils
  • Eosinophils
  • Basophils

  
• Neutrophils


  • AKA POLYMORPHONUCLEAR NEUTROPHILIC (PMN) LEUKOCYTE, represent approximately 50% to 75% of the total peripheral WBCs in adults, 10 to 15 μm in diameter with a nucleus that has between two to five lobes, Azurophilic or primary granules contain antimicrobial products, Specific granules contain lysozyme, lactoferrin, collagenase, gelatinase, and respiratory burst components, Half of the total neutrophil population in peripheral blood is found in a marginating pool adhering to blood vessel walls, the rest circulate freely for approximately 6 to 8 hours
• Eosinophils


  • Approximately 12 to 15 μm in diameter and normally make up between 1% and 3% of the circulating WBCs in a nonallergic person, Number increases in an allergic reaction or in response to certain parasitic infections, Nucleus is usually bilobed or ellipsoidal and is often eccentrically located, Capable of phagocytosis but much less efficient than neutrophils, Can neutralize basophil and mast cell products, Can use cationic proteins to damage cell membranes and kill larger parasites that cannot be phagocytized, Most important role is the regulation of the immune response, including the regulation of mast cell function

  
• Basophils

  • The least numerous WBC found in peripheral blood, representing less than 1% of all circulating WBCs, Slightly larger than RBCs (between 10 to 15 μm in diameter) and contain coarse, densely staining deep-bluish-purple granules that often obscure the nucleus, Constituents of these granules include histamine, cytokines, growth factors, and a small amount of heparin, all of which have an important function in inducing and maintaining allergic reactions, Regulate some T helper (Th) cell responses and stimulate B cells to produce the antibody IgE, Have a short life span of only a few hours
• Eosinophils


  • Can neutralize basophil and mast cell products
  • Can use cationic proteins to damage cell membranes and kill larger parasites that cannot be phagocytized
  • The most important role is the regulation of the immune response, including the regulation of mast cell function