Chapter 20 - Part II The Lymphatic System Elephantiasis Hardening & thickening of skin following swelling of arms, breasts, legs, others (lymphedema) Caused by parasitic worms living in lymphatic system Source: http://www.bio.davidson.edu/Courses/immunology/Students/spring2006/Heeren/eleph_clip_image004.jpg http://www.stanford.edu/class/humbio103/ParaSites2006/Lymphatic_filariasis/Images/Wuchereria_bancrofti_1_DPDX.JPG Elephantiasis mosquitoe take a blood meal & also introduces worm larvae in saliva larvae move into lymphatic system & grow into adults ranging from 1-4? average lifespan = 7 years Filarial worm Lymphatic System Overview Lymphatic system - is a major component of immune system Consists of two semi-independent parts: 1. meandering network of lymphatic vessels returns IF & ?leaked? plasma proteins back into blood (maintains blood volume) 2. lymphoid tissues & organs scattered throughout body resident site for phagocytic cells & lymphocytes Source: www.winnetmag.com/Files/ 3966/Screen_01.GIF Tractor-trailer Interstitial fluid (IF) & Lymph IF - formed as plasma fluid is forced out of capillary bed arteriole end into surrounding tissue spaces not immediately returned to venous circulation (residual time) exchange medium for gases, nutrients, wastes between blood & cells Lymph - interstitial fluid once it has entered lymphatic vessels 3L formed each 24 hrs as interstitial fluid is returned to venous circulation flows toward heart (one-way system) Lymphatic Vessels Present in most tissues except for CNS, bones, teeth, bone marrow CSF drains fluid back into venous circulation lymph vessels are very low pressure (need help like veins) Lymph vessels include (small-to-large): lymphatic capillaries lymphatic collecting vessels lymphatic trunks lymphatic ducts Lymphatic Vessels (con?t) Types: lymphatic capillaries - microscopic, permeable, blind-ended vessels ?weaving? through capillary beds & tissues lymphatic collecting vessels - similar to veins but very thin-walled, contain more valves & anastomose more lymphatic trunks - formed by union of lymphatic vessels named for regions served (e.g. jugular, subclavian, etc.) Lymphatic Vessels (con?t) Types (con?t): lymphatic ducts - two large thoracic region ducts where all lymph is delivered 1. right lymphatic duct - drains right arm, thorax, right side of head (green area) 2. thoracic duct - arises from cisterna chyli (junction of 3 lower body trunks) right & left lumbar trunks (2) & intestinal trunk drains all other body regions (yellow area) Lymphatic Capillaries Characteristics: similar to blood capillaries? but with modifications 1. remarkably permeable 2. loosely joined endothelial overlap each other forming minivalves that are easily opened 3. anchored to collagen so ? interstitial pressure (more fluid) forces flaps open under increased pressure, closes when pressure decreases mini-valve action is one-way allows interstitial fluid to enter lymph capillaries does not allow lymph to escape from lymphatic capillaries Lymphatic Capillaries (con?t) Lymph capillaries open to absorb larger particulate matter during tissue inflammation: 1. cellular debris 2. pathogens (e.g. bacteria, fungi, etc.) 3. cancer cells Allows infectious agents to travel ?trapped? by lymph node immune cells, which cleanse &?examines? debris/cells Lacteals - specialized lymph capillaries present in intestinal mucosa absorbs digested fat in and delivers chyle (fatty lymph) to blood Lymph Transport Lymphatic system lacks an organ that acts as a pump Vessels are low-pressure conduits Uses similar methods as veins to propel lymph 1. pulsations of nearby arteries 2. smooth muscle contractions in lymphatic walls Lymphoid Cells Lymphocytes - main cells involved in immune response B cells & T cells are two main lymphocyte types protect body against foreign antigens antigen - anything body perceives as foreign (not-self) 1. bacteria (and their toxins) 2. viruses 3. incompatible RBCs (transfused) 4. cancer cells Lymphocyte Source: www.mssm.edu/medicine/hematology/lymphocyte.gif www.sirinet.net/~jgjohnso/antibodieaction2.jpg Antibodies & Antigens Lymphoid Cells (con?t) Functions: T cells - perform self-nonself recognition & can attack/destroy foreign cells once activated B cells - activate producing plasma cells, which then secrete antibodies macrophages - phagocytize foreign substances & help activate/recruit T cells dendritic cells - ?spiny cells? that perform Ag-presentation & long-term Ag recognition reticular cells - fibroblast-like cells producing stromal network supporting other cell types in lymphoid organs Dendritic cells Source: www.mayo.edu/research/bmb/esche2.jpg Lymphoid (Lymphatic) Tissue Lymphoid tissue - scattered lymphatic tissue protecting body from foreign entities (e.g. bacteria) sites for resident lymphocytes (inactive & activated) offers surveillance area for macrophages & lymphocytes includes: lymph nodes spleen tonsils & adenoids thymus Lymphoid (Lymphatic) Tissue Types: diffuse lymphatic tissue - scattered reticular tissue elements found in every body organ appear in lamina propria of mucous membranes & lymphoid organs lymphatic follicles (nodules) - solid, spherical bodies consisting of tightly packed reticular elements & cells smaller than lymph nodes do not filter lymph found in isolation (stomach, intestines) & as part of larger lymphoid organs (spleen) Lymph Nodes Types (con?t): lymph nodes - lymphatic tissue imbedded in connective tissue & clustered along lymphatic vessels nodes are immune system activity centers fed by lymphatic vessels node clusters occur near body surface in inguinal, axillary, & cervical regions Functions: 1. filtration - macrophages filter lymph to destroy microorganisms and debris 2. immune system activation - monitor for antigens & mount an attack against them Lymph Node Structure: bean shaped, surrounded by a fibrous capsule no capsule in diffuse lymphatic tissue or lymphatic nodules trabeculae - extend inward from capsule & divides node into compartments lymph sinuses - vessel-like tracts slowing lymph flow through nodes two histologically distinct regions: 1. cortex 2. medulla Lymph Node Cortex - contains lymphoid follicles same follicles seen in diffuse lymphatic tissue & lymphatic nodules germinal centers - follicles loaded with dividing B cells & plasma cells deep cortex houses T cells in transit T cells continuously circulate in blood, lymph nodes, & lymphatics dendritic cells - nearly encircle follicles http://www.healthsystem.virginia.edu/internet/hematology/images/Secondary-lymph-follicle-10x-website-labeled.jpg Lymph Node Structure Medulla: medullary cords - thin cortical extension containing B cells, T cells, & plasma cells medullary sinuses - vessel-like spaces separating medullary cords contain macrophages (phagocytize foreign matter) and reticular cells Lymph Node Circulation Lymph enters via a number of afferent lymphatic vessels Enters a large subcapsular sinus & travels into a number of smaller sinuses Lymph moves through sinuses and exits node at hilus via efferent vessels fewer efferent vessels slows lymph movement in node allows lymphocytes & macrophages time to carry out their protective functions Other Lymphoid Organs Structure: spleen, thymus gland, tonsils, & Peyer?s patches (intestine) lymphatic tissue ?bits? are scattered in connective tissue all are composed of reticular connective tissue Function: help protect body areas not directly served by nodes only lymph nodes filter lymph Spleen Largest lymphoid organ Blood delivered & returned by large splenic artery & vein enter/exit at hilus Functions: 1. lymphocyte proliferation site 2. immune surveillance & response 3. cleanses blood of debris 4. stores RBCs breakdown products 5. site of fetal erythrocyte production 6. stores blood platelets Spleen Structure: surrounded by a fibrous capsule trabeculae - extend inward containing lymphocytes, macrophages, & many erythrocytes two distinct areas: 1. white pulp - around central arteries, contains mostly lymphocytes suspended on reticular fibers involved in immune functions 2. red pulp - venous sinuses & splenic cords (rich in macrophages) disposes of worn-out RBCs & bloodborne pathogens Thymus Bi-lobed organ secreting thymosin & thymopoietin hormones hormones cause T cells to become immunocompetent cortex - densely packed lymphocytes & scattered macrophages medulla - fewer lymphocytes & thymic (Hassall?s) corpuscles size varies with age: 1. infants - found in inferior neck & is large 2. childhood - increases in size & is most active 3. adolescence - stops growing & gradually atrophies Tonsils Simplest lymphoid organs; form a ring of lymphatic tissue around pharynx Tonsil locations: palatine tonsils - either side of posterior end of oral cavity lingual tonsils - lie at base of tongue pharyngeal tonsil - posterior wall of nasopharynx tubal tonsils - surround openings of auditory tubes into pharynx Epithelial tissue overlying tonsil masses invaginates, forming blind-ended crypts that trap/destroy bacteria and particulate matter Aggregates of Lymphoid Follicles & MALT Peyer?s patches - isolated clusters of lymphoid tissue, similar to tonsils found in wall of distal portion of small intestine MALT (mucosa-associated lymphatic tissue) - protects digestive & respiratory systems from foreign matter, composed of: Peyer?s patches, tonsils, and appendix (digestive tract) lymphoid nodules in walls of bronchi (respiratory tract)
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