Chapter 22: The Lymphatic System

(The Lymphatic System) Chapter 22 The Lymphatic System · also referred to as the lymphoid system · includes lymphoid cells, lymphoid tissues and organs, lymph, and lymphatic vessels ↳ lymph is a fluid connective tissue · primary lymphatic tissues: sites where lymphocytes form and mature ↳ red bone marrow, thymus · Secondary lymphatic tissues: sites where lymphocytes are activated ↳ tonsils, spleen, MALT, lymph nodes Functions of the Lymphatic System ① Production, maintenance, and distribution of lymphocytes ② Ensuring stable blood volume and comparable interstitial composition throughout the body ③ Assists in transport of hormones, nutrients, and waste products Major Components of Lymphatic System · Lymph: the fluid contents of lymphatic vessels ↳ consists of interstitial fluid, lymphocytes, and macrophages · Lymphatic Vessels: network of vessels that begin at the peripheral tissues and extend to the veins ↳ close-ended vessels that begin at or pass through lymphatic tissues and organs · Lymphoid Cells: lymphocytes, phagocytes (macrophages), and a small number of other cells Where does Lymph come from? ↳ lymph is interstitial fluid that enters lymphatic vessels ↳ colourless fluid with lots of WBC Lymph · similar to plasma, except: ↳ higher in lymphocytes ↳ rich in fats ↳ high in cells & proteins · 95% water · removes bacteria, certain proteins, transports fats from small intestine, supplies lymphocytes · lymph → lymphatic capillaries → lymphatic trunks/ducts → back to blood Lymphatic Vessels · low pressure vessels · begins with capillaries, capillaries combine to form larger lymphatic vessels · fluid flows through lymph nodes Lymphatic Capillaries · close-ended tubes, present in almost all tissues around the body (except avascular areas) · lined with loosely bound endothelial cells and incomplete (or absent) basement membrane (forms an overlap that acts like a valve) · have thin walls and larger lumenal diameter than blood capillaries Circulation of Lymph · small lymphatic vessels ↳ pale golden colour, carries lymph from lymphatic capillaries toward trunk ↳ 3 layers to walls, but are smaller and more prevalent than veins ↳ small and medium vessels also have valves Circulation of Lymph cont'd · major lymph-collecting vessels ↳ superficial lymphatics: in subcutaneous layer of skin in areolar tissue of mucous membranes and serous membranes ↳ deep lymphatics: accompany deep arteries and veins, supply the skeletal muscles and organs in neck, limbs, trunk, etc. ↳ collecting vessels merge to form larger vessels called lymphatic trunks - lymphatic trunks empty into 2 larger collecting vessels: ↳ thoracic duct ↳ right lymphatic duct Major Lymphatic Vessels · Lymphatic Trunks: Right lymphatic duct ↳ jugular, subclavian, broncho-mediastinal, intestinal, lumbar · 2 Main Ducts: ↳ Thoracic (left lymphatic) duct: receives lymph from the upper left side of the body, and both sides inferior to the pelvis (its larger b/c it supplies everything but the upper right) ↳ Right lymphatic duct: receives lymph from the upper right side of the body, located near the right clavicle Thoracic duct Lymphedema · significant swelling b/c lymphatic drainage is blocked ↳ obstruction of vessels ↳ vessels don't exist · can damage connective tissues in the area and cause permanent damage · common in ankles and feet Lymphoid Cells · lymphocytes are the main cells involved in the immune response · 3 classes of lymphocytes ↳ T-cells (from thymus) ↳ B-cells (from bone marrow) ↳ Natural Killer cells (from bone marrow) · T-cells and B-cells protect the body against pathogens ↳ T-cells: manage the immune response and attack foreign cells ↳ B-cells: produce plasma cells which secrete antibodies to immobilize antigens · Other lymphoid cells: ↳ macrophages: phagocytize foreign substances and activate T-cells ↳ dendritic cells: similar functions to macrophages ↳ reticular cells: support cells for other cells in lymphoid tissues Lymphoid Tissues & Organs · lymphoid tissues: connective tissues dominated by lymphocytes · lymphoid nodules: lymphocytes densely packed in areolar tissue, with germinal center where dividing lymphocytes reside · Tissues: mucosa associated lymphoid tissue (MALT) · Organs: lymph nodes, tonsils, thymus, spleen MALT (mucosa associated lymphoid tissue) · lymphatic nodules associated with mucous membranes · protect the epithelia of the digestive, respiratory, reproductive, and urinary systems · peyer's patches: clusters of aggregated lymphoid nodules in the small intestine · example: appendix and tonsils Tonsils · secondary lymphatic tissue · 5 tonsils: pharyngeal (adenoid), 2 palatine, 2 lingual · tonsillitis: inflammation of the tonsils (especially palatine tonsils) · tonsils reach largest size at puberty then begin to atrophy Thymus · primary lymphoid organ, located in mediastinum (posterior to sternum) · functions: produces thymic hormones ↳ hormone: Thymosin » development of T-cells · most active in childhood; decreases and atrophies in adulthood (involution) · stem cells → T-cells · T-cells mature in cortex of lobule and migrate to the medulla, then enter circulation ↳ blood thymus barrier: separates maturing cells from the circulation · When activated, T-cells divide and produce more T-cells ↳ also secrete cytokines (immune mediators) Spleen · secondary lymphoid organ · largest lymphoid tissue in body · functions: ↳ removes abnormal blood cells and other blood components by phagocytosis ↳ stores iron recycled by blood cells ↳ initiates immune response by B-cells and T-cells (in response to antigens in circulating blood) · made of red pulp (RBC-rich) and white pulp (resembles lymphoid nodule) within a capsule ↳ red pulp contains meshwork of reticular fibers → blood passes through meshwork and enters sinusoids lined with macrophages → sinusoids empty into small veins and then trabecular veins · flow of blood through spleen gives phagocytes time to recognize and engulf damaged or infected cells in the blood, and trigger an immune response Lymph Nodes · small lymphoid organs · 600ish in body, most found in neck, axillae, and groin · functions: ↳ filter and purify lymph ↳ lymphocytes in lymph node attack and destroy pathogens ↳ protect entrances and monitor lymph · Lymphadenitis: "Swollen glands", nodes increase in size in the event of pathogenic invasion/infection due to increased amounts of lymphocytes Paracortex Innate Immunity · non-specific · from physical barriers to phagocytes, NK cells, immune surveillance, interferons, inflammation, and fever · NK cells: detect unfamiliar antigens, respond faster than B-cells or T-cells ↳ generalized attack ↳ perforin and protectin Adaptive Immunity · depends on the response of T-cells and B-cells (specific to antigen) · develop mostly after birth · When an antigen triggers an immune response, it activates both B and T-cells ↳ T-cells respond first to attack antigen, then stimulate B-cells ↳ B-cells differentiate into plasma cells and produce antibodies when needed · Characteristics of Adaptive Immunity: ↳ specificity ↳ versatility ↳ memory ↳ tolerance Types of Adaptive Immunity ① Cell-Mediated Immunity ↳ T-cytotoxic cells, T-helper cells, Regulatory T-cells, Memory T-cells ② Antibody-Mediated (Humoral) Immunity ↳ B-cells ↳ When a B-cell activates it will make copies of itself that are sensitive to a specific antigen ↳ may be active or passive, naturally acquired or artificially acquired: i. naturally acquired active immunity Antibody-mediated (fluids) Cell-mediated (in cells) ii. artificially acquired active immunity iii. naturally acquired passive immunity iv. artificially acquired passive immunity