Lecture 19: Lymphatic System

Lymphatic System Concepts

• Bone marrow and thymus are primary lymphoid organs.
• Bone marrow cells differentiate into lymphoid and myeloid precursors.
• Lymphatics maintain fluid balance, aid fat absorption, and enable immune reactivity.
• Lymphatic flow depends on smooth muscle contractions and body movement.
• Lymph is fluid from cellular secretion composed of 96% water and 4% solids.
• Spleen and lymph nodes are secondary lymphoid organs and antigen response sites.
• The liver, MALT, GALT, BALT and skin are immune reactive areas.

Learning Objectives

• The site, progenitor cells and their differentiated cell types of hematopoiesis, and the destination of immune cells, can be identified.
• Molecules that direct hematopoiesis can be identified
• The different types of increased and decreased leukocytes and their causes can be identified.
• The composition of lymph can be identified.
• The primary functions of the lymphatic system can be identified.
• The movement of lymph and lymphatic drainage are described.
• Afferent and efferent lymphatic vessels and the role of high endothelial venules in immune cell trafficking can be distinguished.
• The cells and functions of the follicle, medulla, and paracortex of the lymph node can be distinguished.
• The location of major lymph nodes, lymphocyte recirculation can be defined.
• The function of the spleen described, and the cells and functions of the red and white pulp of the spleen can be distinguished
• The result of asplenia explained.
• The role of tertiary lymphoid tissues is identified and outlined.
• The primary immune cells and/or structures and their functions in secondary lymphoid tissues can be recognized.

Organs of Immune System

• Primary lymphoid organs support immune cell birth and maturation.
• Secondary lymphoid organs store immune cells, activate them, and facilitate response to infection.
• Secondary lymphoid organs optimize interactions between antigens, antigen-presenting cells, and lymphocytes.
• The blood and lymphatic circulatory systems link all immune organs.

Immune System Cells

• Immune responses arise from coordinated activities throughout the body.
• Hematopoietic stem cells (HSCs) can differentiate into various blood cell types.
• All red and white blood cells during hematopoiesis develop from a pluripotent HSC.
• Hematopoiesis is a highly regulated process which matures cells in different locations.
• The lymphatic system connects primary and secondary lymphoid organs with the circulatory system.
• Colony Forming Units (CFUs) lose self-renewal capacity and create specific blood cell lineages.

Hematopoiesis Drivers

• Hematopoiesis is largely regulated by the presence of growth factors.
• Fetus: Hematopoietic cells move from yolk sac to liver and then to bone marrow.
• After birth SCF is needed for normal blood cell production.
• GM-CSF produces neutrophils, eosinophils and monocytes.
• Interleukin-3, with other growth factors, produces mature red cells, granulocytes, monocytes and platelets.
• Antigen stimulated T cells and cytokine/microbe-activated macrophages help replenish leukocytes.

Bone Marrow

• Bone marrow stromal cells are mesenchymal stem cells and differentiate into osteocytes, adipocytes, chondrocytes, and fibroblasts.
• They produce colony stimulating factors that promote hematopoietic stem cell differentiation.
• Stromal cells in bone marrow and thymus regulate lymphocyte development.
• Bone marrow microenvironment is a complex, 3D structure which contains stem cells.

Lymphoid Cells

• Pluripotent hematopoietic stem cells produce leukocytes, erythrocytes, and platelets.
• Myeloid lineage generates thrombocytic, erythroid, granulocytic, monocytic, and dendritic cells.
• Lymphoid lineage creates lymphocytes like T cells, B cells, and NK cells.

Leukocytes

• Neutrophils are the the majority (50-70%) of the circulating leukocytes
• Neutrophils migrate to infection sites from inflammatory molecules.
• Basophils are nonphagocytic cells with granules that contain basophilic proteins that release histamine.
• Histamine increases blood vessel permeability and smooth muscle activity.
• Mast cells mature only after they leave the blood and play a role in allergy development.
• Eosinophils migrate from the blood into tissue spaces to defend against parasitic worms.

Lymphoid Precursors

• Helper T cells activate B cells to become plasma cells.
• Plasma cells secrete soluble forms of immunoglobulin called antibodies.

Thymus Facts

• The thymus is a bi-lobed primary lymphoid organ
• Stem cells (prothymocytes) originate in bone marrow, and migrate to thymus to become a T cell.
• When cells arrive at the thymus, they aren't technically T cells.
• Cells in the thymus can also become NK cells, dendritic cells, B cells, and even myeloid cells.
• A surface receptor known as Notch commits cells to the T cell lineage.

Cell Destinations

• Common myeloid precursors can become erythrocytes, granulocytes, platelets, megakaryocytes etc
• Common lymphoid precursors can become T Cells, B Cells and NK cells

Adaptive Immunity

• Adaptive immunity is initiated in secondary lymphoid tissues.
• Pathogens, DCs, and macrophages enter via afferent lymph.
• DCs move to T cell areas and present to T cells, which proliferate into effector cells.
• T cells leave via efferent lymph and travel to infected tissue/stimulate B cell → plasma cells.
• Plasma cells secrete antibodies that target infection sites.

Lymph Facts

• Lymph is fluid from cellular secretions, pushed out of capillaries to bathe tissues.
• Lymph consists of 96% water and 4% solids, including proteins, lipids, carbohydrates, nitrogen products, and electrolytes.
• Tissue collects lymph into lymphatic capillaries, directs it to lymph nodes for filtration, then returns fluid to venous circulation.

Lymphatic vessel functions

• Lymphatic vessels maintain fluid balance within the body.
• They aid in digested fat absorption.
• They generate an immune response against pathogens (antigens).
• The flow of plasma is filtered into interstitial space from blood flowing through capillaries and excess fluid, if not reabsorbed, can cause tissue damage.
• The body contains 600-700 lymph nodes in the average human

Lipid Transport and Nodes

• Dietary lipids' transport into circulation differs from sugars' and amino acids' transport.
• Monoglycerides and fatty acids diffuse/enter via transporter proteins across enterocyte membranes.
• Monoglycerides/fatty acids convert into triglycerides in enterocytes and are packaged w/ cholesterol, lipoproteins, other lipids and chylomicrons.
• Chylomicrons are released from enterocytes, taken up by lymphatic vessels, and eventually drain into the venous blood.
• Lymph nodes (LN) filter lymph before returning it to the circulation, and damaged/destroyed LNs do not regenerate.
• Interstitial fluid pressure forces endothelial cells of lymphatic capillaries to open briefly, and surrounding tissue movement drives lymph through the vessels .

Lymphocytes

• B and T cells move in blood and lymph, recirculating continuously.
• Lymphocyte populations survey tissue for infection, remaining in lymph nodes when activated.
• If lymphocytes are not activated in LNs, they return to blood via efferent lymph.
• Afferent vessels carry lymph toward lymph nodes and efferent vessels carry lymph away from lymph nodes.

Immune Cell Entry

• Immune cells enter lymph nodes from tissues via afferent vessels.
• Immune cells enter lymph nodes from blood via high endothelial venules (HEV).
• Lymph nodes are specialized for lymphocyte and antigen encounters.

Lymph Node components

• The cortex contains naïve B cells, follicular dendritic cells, subcapsular macrophages.
• The paracortex contains T cells and dendritic cells.
• The medulla contains macrophages and plasma cells.
• The cortex lymphocytes are mostly B cells, macrophages, and follicular dendritic cells arranged in follicles.
• The paracortex is primarily T cells containing dendritic cells.
• The medulla lymphocytes exit via efferent lymphatics and contain sparsely populated macrophages and plasma cells.

Major Lymph Nodes Function

• Cervical nodes drain head and neck.
• Axillary nodes drain upper arm and upper thoracic area (+ breast).
• Thoracic Cavity nodes drain organs of thorax (heart, lungs).
• Supratrochlear nodes drain area above bend of elbow (lower arm).
• Abdominal nodes drain areas of the GI tract and trunk.
• Pelvic Cavity nodes drain pelvic area including internal genitals.
• Inguinal nodes drain legs and external genitals.

Lymph Movement

• Lymph flows upward, traveling from extremities toward the neck.
• Lymph travels through the thoracic duct and becomes plasma in the bloodstream
• Movement through breathing, skeletal muscle contractions, and segmental contractions.
• Posture changes and osteopathic manipulative medicine (OMM) promote movement.

Lymph Drainage

• The right side of head and neck, the right arm, and the upper right quadrant drains into the Right Lymphatic Duct.
• The Right Lymphatic Duct empties into the Right Subclavian Vein.
• The left side of head and neck, the left arm, the upper left quadrant, the lower trunk, and both legs drains into the Cisterna Chyli.
• Cisterna Chyli is a transient storage of lymph from the abdomen and legs
• Fluid from the Cisterna Chyli then travels through the Thoracic Duct to drain in the Left Subclavian Vein.

Spleen facts

• The spleen defends against blood-borne pathogens.
• The red pulp filters blood and removes old red blood cells and pathogens.
• In hemorrhage cases, the blood is reserved in the red pulp.
• The red pulp is a site of hematopoiesis if bone marrow fails - it contains red blood cells, macrophages, and some lymphocytes.
• Bloodborne antigens captured in the marginal zone is the area between the red and white pulp
• The white pulp contains the periarteriolar lymphoid sheath (PALS), B cells in lymphoid follicles etc.
• PALS contains dendritic and T cells.

Spleen details

• Unlike lymph nodes, it's not supplied by lymphatic vessels and instead transports antigens and lymphocytes through the splenic vein/artery.
• Vascular sinusoids are separated by connective tissue called splenic cords that are in the red pulp.
• Splenic cords store high numbers of plasma cells, dendritic cells, macrophages, and red blood cells.
• The white pulp concentrates B and T cells with - periarteriolar lymphoid sheath (PALS) containing dendritic cells and T cells as well as B cells in lymphoid follicles.

Area of Blood Capture

• The area is between red and white pulp.
• It is the residence for the marginal zone B (MZB) cells.

Asplenia facts

• Congenital asplenia is caused by mutation in a gene for ribosomal protein SA.
• Some people have their spleen removed due to infection or trauma.
• those without a spleen are susceptible to infections with encapsulated bacterial SHINS (Streptococcus pneumonia, Haemophillus influenza, or Neisseria meningitidis, Salmonella.)
• These individuals lack IgM to activate complement for the initial immune reaction.
• These individuals are more prone to sepsis.
• Immunization is important in these individuals who are also more vulnerable to infections

Secondary Lymph Tissue

• Skin and mucosal membranes (digestive, respiratory & urogenital tracts) are secondary lymphoid organs.
• They are collectively referred to as mucosa-associated lymphoid tissue (MALT). These areas are major sites of entry for pathogens and are lined with epithelial cells.
• The skin and mucosal membranes represent a surface area of over 400 m2.
• Lymphoid nodules in the tonsils are similar to Peyer's Patches in the Gut.

Other Areas

• Vertebrate skin has epidermis and dermis.
• The epidermis, is known as the stratum corneum.
• The dermis, is rich in collagen, and elastin fibers

Tertiary Lymph tissue

• Tertiary lymphoid tissue is a site of active infection and immune activity in the lung, liver, brain, and skin.
• Activated lymphocytes return to these areas as effector or tissue-resident memory cells.

Liver Functions

• Breaks down fats, converts glucose to glycogen.
• Makes proteins including acute phase proteins, and stores vitamins/minerals.
• Contains Kupffer cells that capture/remove antigens.

Liver Details

• Liver sinusoids are lined by liver sinusoidal endothelial cells (LSEC).
• Lymphocytes passing through sinusoids are in contact with endothelial cells.
• The Space of Disse contains hepatic stellate cells (HSC) that store fat.
• Resident DC can capture the antigen, process and present to the T cells in the sinusoids and HSC are activated in damaged liver resulting in collagen production, fibrosis and ultimately cirrhosis.

Granulocytes

• granulopoiesis is production of granulocytes.
• Polymorphonuclear leukocyte (PMN) = Granulocyte
• Granulocytes – white blood cells with multi-lobular nuclei and cytoplasmic granules
• Neutrophils, basophils, eosinophils

Leukocytes

• Increased leukocytes are categorized as "philia"
• Decrease leukocytes are categorized as "penia"

Leukocytosis and causes

• Leukocytosis is defined as an increase in total number of WBCs/liter where blood exceeds higher limit of normal in peripheral blood
• Leukocytosis > 11,000/cmm, where a normal range : 5,000-11,000/cmm
• Caused by any of the following:
• Neutrophilia, lymphocytosis, monocytosis, eosinophilia, or basophilia

Neutrophilia Details

• It an increase in neutrophils with infections and can triggered by acute bacterial infections and acute inflammation.
• May also be due to myocardial infarction or burns.
• Eosinophilia is an increase in neutrophils and can be triggered by allergic disorders or parasitic infestations

White Blood Cells issues

• Basophilia is an increase in basophils and can be triggered by myeloproliferative disease, allergic reactions or chronic inflammation
• Monocytosis is an increase in monocytes and can be triggered by bacterial endocarditis, chronic inflammation and Lupus

Blood Infections

• Lymphocytosis is an increase in lymphocytes and caused by infections (viral, bacterial or parasitic)
• Viral Infections include Mononucleolis, CMV, mumps and measles
• Bacterial include Whooping Cough, TB and brucellosis
• Autoimmune disorders include Lupus
• Others Enocrine - Grave's disease and Malignancy-Chronic lymphocytic leukemia

Clinically significant reduction in neutrophils

• Clinically significant Neutropenia is the reduction of making individuals susceptible to bacterial and fungal infections is caused by: - Drug toxicity - idiosyncratic reaction to drugs, or - antibody mediated destruction of mature neutrophils

Lymphopenia

• Lymphopenia is the decrease in circulating lymphocytes caused by: - Drugs such as Corticosteroids, cyclophosphamide - Autoimmune destruction for example in Systemic lupus erythematosus (SLE) - Immuno-deficiency syndrome - Di George syndrome (T-cell deficiency) or Acquired immunodeficiency syndrome (AIDS)
• Severe combined immunodeficiency.
• Endocrine - Cushing's syndrome

Leukocytes Lifetime

• Neutrophil 6 hours - few days
• Eosinophil 8-12 days,
• Basophil A few hours to a few days
• Mast cell - Several months
• Macrophages Activated: only days -weeks, Tissue Resident: months to years
• Dendritic cells Similar to macrophages
• RBC Approximately Four Months
• T-cell Years for memory cells
• B-cell Weeks for activated