The Lymphatic System

Lymphatic System

Overview

• The lymphatic system, also known as the lymphoid system, is composed of organs, vessels, cells, and tissues.

• It helps the body maintain homeostasis.

• The body faces daily challenges from potential threats such as UV light, pathogens, extreme cold, toxins, and physical trauma.

Microbes and Pathogens

• Microbes (bacteria, viruses, protists) are present everywhere in the environment.

• They are found in soil, water, air, food, and houses.

• Some microbes are pathogens, necessitating defense mechanisms.

Components of the Lymphatic System

• Extensive network of branching vessels.

• Lymph nodes: structures along vessels filled with a mesh of fibers and WBCs.

• Bone marrow.

• Other organs: thymus, spleen, appendix, tonsils.

Lymph

• Lymph is the fluid carried by lymphatic vessels.

• It originates from blood pushed into tissues surrounding capillaries.

• Lymph is collected into small lymphatic vessels, which merge into larger ones and re-enter the bloodstream in the shoulder areas.

• Lymph nodes swell during illness or cancer due to the accumulation of WBCs.

Body's Defenses

• Nonspecific defenses: anatomical barriers and mechanisms that prevent or slow pathogen entry or attack them without distinguishing between threats.

• Specific defense: The immune system involves the lymphatic system.

Specific Defense and Lymphocytes

• The primary function of the lymphoid system is the production, maintenance, and distribution of lymphocytes.

• Lymphocytes circulate in the blood and peripheral tissues, ready to attack antigens.

Lymphatic Vessels

• Lymphatic vessels carry lymph from peripheral tissue to the venous system.

• Components:

  • Lymphatic capillaries

  • Small lymphatic vessels

  • Major lymph-collecting vessels

Lymphatic Capillaries

• Smallest lymphatic vessels that branch through peripheral tissues.

• Differ from blood capillaries:

  • Originate as pockets rather than continuous tubes.

  • Have larger diameters.

  • Have thinner walls.

  • Have a flattened outline in sectional view.

• Lined with endothelial cells; basal lamina is incomplete or absent.

• Endothelial cells overlap, allowing entry of fluids, solutes, viruses, bacteria, and cell debris while preventing their return into intercellular spaces.

Small Lymphatic Vessels

• Lymph flows from capillaries into larger vessels that lead to the body’s trunk.

• Walls are comparable to veins and contain valves to prevent backflow.

• Pressures within the lymphoid system are minimal.

• Normal lymph flow depends on valves and muscular contractions.

Major Lymph-Collecting Vessels

• Two sets:

  • Superficial lymphatics: located in the subcutaneous layer, mucus membranes lining the GI, respiratory, urinary, and reproductive tracts, and areolar tissues lining the pleural, pericardial, and peritoneal cavities.

  • Deep lymphatics: accompany deep arteries and veins supplying skeletal muscle, visceral walls, and organs in the neck, limbs, and trunk.

• Superficial and deep lymphatics converge to form larger vessels called lymphatic trunks.

• Trunks empty into two larger vessels: the thoracic duct and the right lymphatic duct.

Thoracic Duct

• Collects lymph from the body inferior to the diaphragm and the left side of the body superior to the diaphragm.

• Begins at the L2 vertebra as an expanded, sac-like chamber called the cisterna chyli.

• The cisterna chyli receives lymph from the inferior abdomen, pelvis, and lower limbs via the right and left lumbar trunks and the intestinal trunk.

• Inferior segment lies anterior to the vertebral column, ascending from L2, it passes posteriorly and to the left of the diaphragm alongside the aorta to the level of the left clavicle.

• After collecting lymph from the left bronchomediastinal trunk, left subclavian trunk, and the left jugular trunk, it empties into the left subclavian vein near the left internal jugular vein.

Right Lymphatic Duct

• Collects lymph from the right side of the body superior to the diaphragm.

• Formed from the merging of the right jugular, right subclavian, and right bronchomediastinal trunks near the right clavicle.

• Empties into the right subclavian vein.

Lymphedema

• Blockage of lymphatic drainage from a limb.

• Interstitial fluids accumulate, causing the limb to become swollen and distended, which may become permanent.

• It does not pose a major threat to life, but carries the risk of infection in the affected area because interstitial fluids are stagnant, leading to accumulation of toxins or pathogens.

Lymphocytes

• Three classes:

  • T (thymus-dependent) cells

  • B (bone marrow-derived) cells

  • NK (natural killer) cells

T Cells

• Make up approximately 80% of lymphocytes.

• Types:

  • Cytotoxic T cells: attack foreign cells or virus-infected body cells directly; involved in cell-mediated immunity.

  • Helper T cells: stimulate the activation of both T and B cells.

  • Suppressor T cells: inhibit the activation and function of T and B cells.

B Cells

• Account for 10-15% of lymphocytes.

• When stimulated, B cells differentiate into plasma cells, responsible for the production and secretion of antibodies (immunoglobulins).

• Antibodies are proteins that bind to antigens (foreign compounds invading the body), and help destroy them; this is the process of humoral immunity.

NK Cells

• Account for 5-10% of lymphocytes.

• Attack foreign cells, viral-infected body cells, or cancer cells.

• Involved in the immunological surveillance of peripheral tissues.

Lymphocyte Production

• Lymphopoiesis involves the bone marrow, thymus, and peripheral lymphoid tissue.

• Initiating in the bone marrow, hemocytoblasts (pluripotent stem cells) generate lymphoid stem cells that form lymphocytes.

• One group of lymphoid stem cells remains in the bone marrow and produces immature B cells and NK cells. As they mature, B cells and NK cells enter the bloodstream and migrate to peripheral tissues (B cells to lymph nodes or spleen and NK cells to all peripheral tissue).

• A second group of lymphoid stem cells migrates to the thymus gland where they develop and replicate under the influence of thymus hormones.

• They are then released into the bloodstream and travel to the bone marrow, peripheral tissues, and lymphoid tissues (spleen).

Lymphoid Tissues

• Connective tissues dominated by lymphocytes.

• Lymphoid nodules occur in large clusters deep to epithelia lining the respiratory tract (tonsils), and digestive (MALT) and urinary tracts.

• Each is approximately a millimeter in diameter, lacks a fibrous capsule, and has a central zone called the germinal center, which contains dividing lymphocytes.

MALT

• Mucosa-Associated Lymphoid Tissue; collection of lymphoid tissues linked to the digestive system.

• Clusters of nodules deep to the lining of the intestine are called Peyer’s patches.

• The walls of the appendix also contain many lymphoid nodules.

Tonsils

• Large lymphoid nodules in the walls of the pharynx.

• Left and right palatine tonsils are located at the posteroinferior oral cavity where it meets the pharynx.

• A single pharyngeal tonsil, called the adenoid, lies in the posterosuperior wall of the nasopharynx.

• A pair of lingual tonsils lie deep to the epithelium covering the base of the tongue.

• Tonsillitis is the condition of inflamed swollen tonsils due to infection; a tonsillectomy may be the treatment of choice if chronic.

Lymphoid Organs

• Unlike lymphoid tissues, lymphoid organs have a fibrous capsule.

• Include lymph nodes, thymus, and the spleen.

Lymph Nodes

• Small in size (1-25mm) and shaped like a kidney bean.

• Consist of a dense connective tissue capsule with collagen fibers extending interiorly from it, called trabeculae.

• Two sets of lymphatic vessels connect to each lymph node: afferent lymphatics carry lymph to the node from the peripheral tissue and penetrate the capsule opposite the hilum (indentation of the node).

• Efferent lymphatics attach at the hilum and carry lymph away from the node to the venous circulation.

Lymph Flow

• Lymph delivered to the node by afferent lymphatics enters the subcapsular space, filled with a mesh formed from reticular fibers, macrophages, and dendritic cells.

• It is filtered here and flows to the outer cortex, which contains B cells.

• Lymph then enters sinuses in the deep cortex, dominated by T cells.

• Finally, it flows into the core of the node called the medulla, which contains B cells and plasma cells colonized into elongated masses called medullary cords.

• The lymph then exits the node via efferent lymphatics.

Lymph Node Function

• Each node functions as a purifying filter before the lymph reaches the venous system.

• At least 99% of antigens in the lymph are removed during this process.

• The nodes may become swollen and tender due to increased activity of the macrophages and lymphocytes in response to local infections.

• Chronic or excessive enlargement of the lymph nodes in response to bacterial or viral infections or cancer constitute lymphadenopathy.

• Lymph node biopsies are common if cancer is suspected and may determine if metastasis occurred.

Thymus

• Located in the mediastinum; largest in an infant to young child and starts to decrease in size after puberty.

• This decrease in size, called involution, makes elderly individuals more susceptible to disease.

• The thymic capsule divides it into two thymic lobes, which are divided into smaller lobules.

• Each lobule consists of a dense outer cortex, where immature T cells are located, and a central medulla, where the T cells mature.

Spleen

• In the adult, it is the largest collection of lymphoid tissue in the body.

• Functions:

  • Remove abnormal or old blood cells via phagocytosis.

  • Stores iron recycled from RBCs.

  • Initiates the immune response by B cells and T cells in response to antigens circulating in the blood.

Anatomy of the Spleen

• Located inferior to the stomach in the LUQ and is the size of a baseball.

• Highly vascular and soft in consistency.

• Contains a hilum, groove between the gastric and renal areas where lymphatic vessels enter.

• Surrounded by a capsule containing collagen and elastic fibers.

• Within the capsule is the pulp: red pulp contains large amounts of RBCs and white pulp resembles lymphoid nodules.

Blood Flow in the Spleen

• The splenic artery enters at the hilum and branches into trabecular arteries and capillaries into the red pulp.

• The red pulp contains sinuses lined with macrophages and lymphocytes that destroys damaged or infected cells. From there the blood enters trabecular veins that leave the spleen.

• Due to the fragility of the spleen, it can easily become lacerated upon trauma. This may lead to massive internal bleeding (medical emergency) and warrants an immediate splenectomy.

Nonspecific Defenses

• The body has two nonspecific lines of defense:

  1. External barriers: physical barriers that prevent infection; include:

     • Skin: forms a tough outer layer that most bacteria and viruses cannot penetrate.

     • Mucous membranes: found in the respiratory and digestive tracts, composed of ciliated cells and cells that secrete mucus which traps bacteria, dust, and other particles; once trapped, particles are moved by beating cilia toward the mouth or nose for evacuation.

     • Secretions: sweat, saliva, skin oils, and tears contain antimicrobial enzymes (lysozyme); stomach acid kills most of the bacteria swallowed.

  2. Internal Defenses: If a microbe does penetrate the external barriers, they are met by either white blood cells (WBC’s) or defensive proteins.

     • The white blood cells are produced in the bone marrow and circulate throughout the blood engulfing microbes, diseased cells or debris from dead cells. These phagocytes are the “first line of defense” against pathogenic invasion and include:

       1. Microphages: Neutrophils and eosinophils.

       2. Macrophages: Monocytes and their derivatives.

Other Internal Defenses

• Natural Killer cells (NK cells) attack bacteria, virus-infected body cells, and cancer cells by adhering to the target cell, secreting destructive proteins called perforins from the golgi apparatus. The perforins are released from the NK cell via exocytosis and attach to the plasma membrane of the foreign or diseased cell. It then penetrates the plasma membrane, causing the cell to burst.

Defensive Proteins

• Interferons:

  • Produced by activated lymphocytes, macrophages, and virus-infected body cells that bind to the membranes of uninfected cells.

  • Stimulate healthy cells to produce antiviral proteins to inhibit viral reproduction within the cell.

• Complement proteins:

  • Circulate in the blood and attach to the membranes of microbes causing them to lyse or to be recognized faster by the white blood cells.

Inflammation

• The body’s response to injured tissues and another type of nonspecific defense.

• Symptoms: redness, heat, swelling, and pain.

• A laceration of the skin causes the damaged skin cells to release a chemical called histamine.

• Histamine causes nearby blood vessels to dilate causing a large amount of blood to enter the area of injury, causing swelling.

• The blood carries platelets to clot the area, WBC’s to clean the area, and extra oxygen and cells to repair the area.

• Injured cells also secrete prostaglandins which also increase the blood flow to the injured area causing it to become red and warm. They also are the “pain” producing chemicals.

• The injured cells may also secrete pyrogens, chemicals that stimulate a fever, in order to discourage bacterial growth.

• Taking aspirin or ibuprofen (NSAIDs), decrease inflammation and therefore swelling and fever. They also block prostaglandin production to reduce pain.

• Pain and inflammation is a good thing, taking these drugs just “mask” the pain temporarily, they are not intended for long-term use because they are not curative!