The Lymphatic and Immune Systems 21 PART 1 The Lymphatic and Immune Systems

The Lymphatic and Immune Systems Lymphatic system Main function is to return excess tissue fluid to blood vascular system Lymphatic vessels collect tissue fluid

The Lymphatic and Immune Systems Protects our bodies from foreign organisms Confers immunity to disease Main components Lymphocytes, lymphoid tissue, and lymphoid organs

Lymphatic vessels collect tissue fluid from loose connective tissue The Lymphatic System Lymphatic vessels collect tissue fluid from loose connective tissue Carry fluid to great veins in the neck Once tissue fluid is within lymphatic vessels it is termed lymph Lymph flows only toward the heart

Functions of Lymphatic Vessels Collect excess tissue fluid and blood proteins Return tissue fluid and blood proteins to bloodstream

Orders of Lymphatic Vessels Lymph capillaries Smallest lymph vessels First to receive lymph Are highly permeable vessels Collecting lymphatic vessels Collect from lymph capillaries Lymph nodes are scattered along collection vessels

Orders of Lymphatic Vessels Lymph trunks Collect lymph from collecting vessels Lymph ducts Empty into veins of the neck

Lymphatic Capillaries Located near blood capillaries Receive tissue fluid from CT Increased volume of tissue fluid Minivalve flaps open and allow fluid to enter High permeability of lymphatic capillaries allows entrance of Tissue fluid and protein molecules Bacteria, viruses, and cancer cells

Lymphatic Capillaries Lacteals Are specialized lymphatic capillaries Are located in the villi of the small intestines Receive digested fats Fatty lymph is termed chyle

Figure 21.1 Distribution and special features of lymphatic capillaries. Venous system Arterial system Heart Lymphatic system: Lymph duct Lymph trunk Lymph node Collecting lymphatic vessels, with valves Lymphatic capillary Blood capillaries Filaments anchored to connective tissue Endothelial cell Flaplike minivalve Tissue fluid Tissue cell Fibroblast in loose connective tissue Blood capillaries Lymphatic capillaries Structural relationship between a capillary bed of the blood vascular system and lymphatic capillaries Lymphatic capillaries are blind-ended tubes in which adjacent endothelial cells overlap each other, forming flaplike minivalves. 10

Collecting Lymphatic Vessels Accompany blood vessels Composed of the same three tunics as blood vessels Contain more valves than veins do Helps direct the flow of blood

Collecting Lymphatic Vessels Flow of lymph is not aided by the heartbeat Lymph is propelled by three weaker mechanisms Bulging of skeletal muscles Pulsing of nearby arteries Tunica media of the lymph vessels

Cleanse the lymph of pathogens Human body contains around 500 Lymph Nodes Cleanse the lymph of pathogens Human body contains around 500 Superficial lymph nodes located in Cervical region Axillary region Inguinal region

Lymph Nodes Deep lymph nodes are Tracheobronchial lymph nodes Aortic lymph nodes Iliac lymph nodes

Entrance of right lymphatic duct into vein Cervical nodes Figure 21.2 General distribution of collecting lymphatic vessels and regional lymph nodes. Internal jugular vein Regional lymph nodes Entrance of right lymphatic duct into vein Cervical nodes Entrance of thoracic duct into vein Axillary nodes Thoracic duct Cisterna chyli Aorta Inguinal nodes Collecting lymphatic vessels Drained by the right lymphatic duct Drained by the thoracic duct 15

Microscopic Anatomy of a Lymph Node Fibrous capsule Surrounds lymph nodes Trabeculae Connective tissue strands extend inward to divide lymph node into segments

Microscopic Anatomy of a Lymph Node Lymph enters convex aspect of a lymph node through Afferent lymphatic vessels Lymph exits a lymph node at the hilum through Efferent lymphatic vessels

Figure 21.3a Structure of a lymph node. Afferent lymphatic vessels Cortex Lymphoid follicle Germinal center Subcapsular sinus Efferent lymphatic vessels Hilum Medulla Medullary cord Medullary sinus Capsule Trabeculae Longitudinal view of the internal structure of a lymph node and associated lymphatics 18

Figure 21.3b Structure of a lymph node. Lymphoid follicles Trabecula Subcapsular sinus Capsule Medullary cords Medullary sinuses Photomicrograph of part of a lymph node (72) 19

Figure 21.3c Structure of a lymph node. Macrophage Reticular cells on reticular fibers Lymphocytes Medullary sinus Reticular fiber Reticular tissue within the medullary sinus (690) 20

Collecting lymphatic vessels converge to form lymph trunks Five major lymph trunks Lumbar trunks Receive lymph from lower limbs Intestinal trunk Receives chyle from digestive organs Bronchomediastinal trunks Collects lymph from thoracic viscera

Five major lymph trunks (continued) Subclavian trunks Receive lymph from upper limbs and thoracic wall Jugular trunks Drain lymph from the head and neck

Figure 21.4a The lymph trunks and ducts. Internal jugular veins Right jugular trunk Esophagus Right lymphatic duct Trachea Left jugular trunk Right subclavian trunk Left subclavian trunk Right subclavian vein Right broncho- mediastinal trunk Brachiocephalic veins Left subclavian vein Superior vena cava Entrance of thoracic duct into vein Left broncho- mediastinal trunk Azygos vein Thoracic duct Ribs Cisterna chyli Right lumbar trunk Hemiazygos vein Left lumbar trunk Inferior vena cava Intestinal trunk Major lymphatic trunks and ducts in relation to veins and surrounding structures, anterior view 23

Figure 21.4b The lymph trunks and ducts. Esophagus Trachea Aorta Azygos vein Vertebral body Left broncho- mediastinal trunk Sympathetic trunk Thoracic duct Cisterna chyli Crus of the diaphragm Inferior vena cava Thoracic duct along the posterior thoracic wall 24

Lymph Ducts Cisterna chyli Thoracic duct Located at the union of lumbar and intestinal trunks Thoracic duct Ascends along vertebral bodies Empties into venous circulation Junction of left internal jugular and left subclavian veins Drains three-quarters of the body

Lymph Ducts Right lymphatic duct Empties into right internal jugular and subclavian veins

Recognizes specific foreign molecules Destroys pathogens effectively The Immune System Recognizes specific foreign molecules Destroys pathogens effectively Key cells of the immune system Are termed lymphocytes

Includes lymphoid tissue and lymphoid organs Lymphoid organs The Immune System Includes lymphoid tissue and lymphoid organs Lymphoid organs Lymph nodes Spleen Thymus Tonsils Aggregated lymphoid nodules Appendix

Infectious organisms trigger an inflammatory response Lymphocytes Infectious organisms trigger an inflammatory response Organisms are attacked by Macrophages, then lymphocytes

Lymphocytes effectively recognize a specific foreign molecule Antigens are any molecules inducing a response from a lymphocyte

Cytotoxic T lymphocytes B lymphocytes and T lymphocytes are the two main classes of lymphocytes Cytotoxic T lymphocytes Attack foreign cells directly Bind to antigen-bearing cells Perforate cell membrane Signal cell to undergo apoptosis Destroy virus-infected cells and some cancer cells

Lymphocytes B lymphocytes Become plasma cells Plasma cells secrete antibodies Mark cells for destruction by macrophages Respond primarily to bacteria and bacterial toxins

Figure 21.5 Lymphocyte function. Target cell, bearing antigen Dead target cell T lymphocyte Antigen 1 T lymphocyte binds to target cell, secretes proteins that lyse the cell’s membrane, and signals the cell to die. T lymphocyte detaches from target cell. 2 3 Target cell dies by apoptosis. Action of cytotoxic T lymphocyte B lymphocyte Plasma cell Antibodies Bacterium Surface antigen Macrophage 1 B lymphocyte gives rise to plasma cell, which secretes antibodies. Antibodies bind to antigens on bacteria, marking the bacteria for destruction. 2 Antibody-coated bacteria are avidly phagocytized. 3 Differentiation and activity of B lymphocyte 33

Lymphocyte Activation Lymphocytes originate in bone marrow Some travel to the thymus gland T lymphocytes Some stay in bone marrow B lymphocytes

Lymphocyte Activation Activated lymphocytes Are able to recognize a unique antigen Gain immunocompetence Travel through bloodstream Meet and bind to a specific antigen Proliferate rapidly

Lymphocyte Activation During activation Lymphocyte is presented its antigen by A macrophage or A dendritic cell

Lymphocyte Activation Both T and B lymphocytes produce clones of Effector lymphocytes Respond immediately, then die Memory cells Wait until the body encounters the antigen again Basis of acquired immunity Prevent subsequent infections of the same illness

Figure 21.6 Differentiation, activation, and recirculation of lymphocytes. Primary lymphoid organs (red bone marrow and thymus) Secondary lymphoid organs (lymph nodes, spleen, etc.) Red bone marrow 1 Origin • Both B and T lymphocyte precursors originate in red bone marrow. Lymphocyte precursors 2 Maturation • Lymphocyte precursors destined to become T cells migrate (in blood) to the thymus and mature there. • B cells mature in the bone marrow. Thymus • During maturation, lymphocytes develop immunocompetence and self-tolerance. Red bone marrow 3 Seeding secondary lymphoid organs and circulation • Immunocompetent but still naïve lymphocytes leave the thymus and bone marrow. • They “seed” the secondary lymphoid organs and circulate through blood and lymph. Antigen 4 Antigen encounter and activation Lymph node • When a lymphocyte’s antigen receptors bind its antigen, that lymphocyte can be activated. 5 Proliferation and differentiation • Activated lymphocytes proliferate (multiply) and then differentiate into effector cells and memory cells. • Memory cells and effector T cells circulate continuously in the blood and lymph and throughout the secondary lymphoid organs. 38

Most important tissue of the immune system Two general locations Lymphoid Tissue Most important tissue of the immune system Two general locations Mucous membranes of Digestive, urinary, respiratory, and reproductive tracts Mucosa associated lymphoid tissue (MALT) Lymphoid organs (except thymus)

Figure 21.7 Mucosa associated lymphoid tissue (MALT). Intestine Muscle layers Lumen Mucous membrane lining small intestine Lymphoid follicle Lumen of intestine Germinal center Mucosa of small intestine (7) 40

Primary lymphoid organs Bone marrow Thymus Secondary lymphoid organs Lymph nodes, spleen, tonsils Aggregated lymphoid nodules Appendix Designed to gather and destroy infectious microorganisms and to store lymphocytes

Figure 21.8 Lymphoid organs. Tonsils (in pharynx) Thymus (in thorax; most active during youth) Spleen (curves around left side of stomach) Aggregated lymphoid nodules (in small intestine) Appendix 42

Thymus Site where immature lymphocytes develop into T lymphocytes Secretes thymic hormones Most active in childhood Functional tissue atrophies with age

Composed of cortex and medulla Thymus Composed of cortex and medulla Medulla contains thymic corpuscles Differs from other lymphoid organs Functions strictly in lymphocyte maturation Arises from epithelial tissue

Thymus, located in the superior mediastinum Figure 21.9 The thymus. Capsule Cortex Thymus Medulla Thymic corpuscle Thymus, located in the superior mediastinum Micrograph of thymic tissue 45

Lymph Nodes Function Lymph percolates through lymph sinuses Most antigenic challenges occur in lymph nodes Antigens destroyed and activate B and T lymphocytes

Largest lymphoid organ Two main blood-cleansing functions Spleen Largest lymphoid organ Two main blood-cleansing functions Removal of bloodborne antigens Removal and destruction of old or defective blood cells Site of hematopoiesis in the fetus

Spleen Destruction of antigens Site of B cell maturation into plasma cells Phagocytosis of bacteria and worn-out RBCs, WBCs, and platelets Storage of platelets

Spleen White pulp Thick sleeves of lymphoid tissue Bloodborne antigens are destroyed as they activate the immune response Provides the immune function of the spleen

Spleen Red pulp Surrounds white pulp Composed of Venous sinuses Splenic cords Responsible for disposing of worn-out RBCs

Figure 21.10a Structure of the spleen. Splenic artery Splenic vein Hilum Diagram of the spleen, anterior view 51

Figure 21.10b Structure of the spleen. Capsule Trabecula Splenic cords Splenic sinusoids Arterioles and capillaries Red pulp White pulp Central artery Splenic artery Splenic vein Diagram of spleen histology 52

Figure 21.10c Structure of the spleen. Diaphragm Spleen Adrenal gland Left kidney Splenic artery Pancreas Photograph of the spleen in its normal position in the abdominal cavity, anterior view 53

Figure 21.10d Structure of the spleen. Capsule White pulp Red pulp Photomicrograph of spleen tissue (30). The white pulp, a lymphoid tissue with many lymphocytes, is surrounded by red pulp containing abundant erythrocytes. 54

Simplest lymphoid organs Four groups of tonsils Palatine, lingual, pharyngeal, and tubal tonsils Arranged in a ring to gather and remove pathogens Underlying lamina propria consists of MALT

Figure 21.11 Histology of the palatine tonsil. Pharyngeal tonsil Palatine tonsil Lingual tonsil Germinal centers in lymphoid follicles Tonsillar crypt 56

Aggregated Lymphoid Nodules and Appendix MALT—abundant in walls of intestines Fight invading bacteria Generate a wide variety of memory lymphocytes Aggregated lymphoid nodules (Peyer’s patches) Located in the distal part of the small intestine Appendix—tubular offshoot of the cecum

Figure 21.12 Aggregated lymphoid nodules. Aggregated lymphoid nodules (Peyer’s patch) Smooth muscle in the intestinal wall 58