Overview of the Lymphatic System

The lymphatic system is composed of tissues and organs that play important roles in the body’s immune system and ability to provide defense, fight infection and stimulate repair.

Both bone marrow and the thymus are considered to be the primary lymphoid organs.

Primary Lymphoid Organs

Bone Marrow

Bone marrow is the site of hematopoietic stem cell division which gives rise to the erythrocytes (Red Blood Cells, RBCs), leukocytes (White Blood Cells, WBCs) and platelets.  RBCs are involved in oxygen delivery and gas exchange within the cardiovascular system.  WBCs are an important component of the lymphatic system and provide defense against pathogens and toxins as well as play a role in tissue repair.

White Blood Cells

WBCs include neutrophils, eosinophils, macrophages and monocytes that are important phagocytic cells that provide innate (non-specific) immune responses.  These phagocytic WBCs also play key roles in stimulating the adaptive (specific) immune response by other members of the WBC family (B and T lymphocytes) cells.  The basophils and mast cells are WBCs that orchestrate the inflammatory response which can help to facilitate defense and tissue repair.  basophils and mast cells that are important

Thymus

The thymus is considered to be a primary lymphoid organ as it is the location of T lymphocyte maturation.  The thymus is in the anterior mediastinum just behind the sternum and is about 5cm at birth increasing to 50gram during adolescence, followed by atrophy during adulthood eventually being replaced by fatty tissue.  The thymus is divided into a cortex and medulla, in which the outer cortex contains thymocytes (immature T cells) and specialized epithelial cells that facilitate the maturation of developing thymocytes.  Thymosin hormones have immunoregulatory functions and stimulate the T cell maturation process.

T cell maturation: Initially, immature T cells migrate from the bone marrow to the thymus cortex where specialized cortical epithelial cells play a role in the positive selection of T cells.  During this stage of maturation, T cells are challenged with self antigens displayed on MHC complexes.  The T cells with T cell receptors (TCRs) that are not able to bind the MHC complexes at all, will go through “death by neglect” and go through apoptosis.  T cells that pass this positive selection test, progress further into the thymus, and in the medulla, the T cells go through a negative selection process, in which T cells that bind too strongly to self-antigens that are displayed on MHC complexes will go through apoptosis.  T cells mature during this process and are only capable of expressing either CD4 or CD8 surface proteins. The T cells  that express CD8 proteins recognize MHC Class I complexes and will mature to become CD8 Cytotoxic T cells. T cells that express CD4 proteins recognize MHC Class I complexes will mature to become CD4 Helper T cells. recognize MHC Class I or II complexes.   After passing both positive and negative selection tests, T cells are termed self-tolerant and self-MHC restricted.  These surviving T cells are now termed naive, until they are challenged with a foreign antigen and launch a specific immune response.

Secondary Lymphoid Organs

Secondary lymphoid organs include: the spleen, lymph nodes and mucosa-associated lymphoid tissues (MALT) and tonsils.

Spleen

The spleen is fist-sized, ranging from 7-14cm long, approximately 200g and is the largest of the secondary lymphoid organs.  The spleen is directly under the left portion of the diaphragm, lateral to the stomach and pancreas, and ventral to the adrenal gland and kidney at the level of the 10th rib.

The spleen has many functions.  The spleen is important in hematopoiesis (particularly during fetal life) and RBC recycling, screens for blood-borne pathogens and houses T and B cell lymphocytes that enable specific (adaptive) immune responses.  The spleen filters blood for pathogens and old/damaged/infected RBCs.  The spleen stores RBCs, WBCs and platelets which can be released during times of emergency.  For example, approximately 240mL of RBCs can be released in the event of hypovolemia and/or hypoxia.

The spleen is often described by its red and white pulp zones which are coloured by the types of cells found in each.  The red pulp contains neutrophils, monocytes, macrophages, dendritic cells, and T cells The red pulp zone recycles old, infected and/or dysfunctional RBCs.  The white pulp area contains T and B lymphocytes with each type in specific areas, and provides specific (adaptive) immune defense.

Lymph Nodes

A key functional component of the lymphatic system are the lymphatic vessels that transport lymph fluid from the interstitial spaces of the body into lymph nodes, where the fluid is screened by lymphocytes for pathogens, prior to being directed into the subclavian veins.

There are approximately 450 lymph nodes found at sites throughout the body, where afferent lymphatic vessels converge, bringing lymph fluid to be checked for pathogens, cancerous cells, debris and toxins. Clusters of lymph nodes are found in the armpit region (axillary lymph nodes), head and neck (cervical lymph nodes) and in the groin (inguinal lymph nodes). Notably, the central nervous system (CNS) lacks lymph nodes, though contains the blood-brain barrier, as well as lymphatic drainage vessels that enter cervical lymph nodes.

Each lymph node contains lymphoid lobules full of lymphocytes and antigen-presenting cells (APCs), such as dendritic cells and macrophages.  Lymph nodes are small, approximately 1.5cm in size, though ranging from 0.2-2.5cm, and oval or kidney-like in shape.

Each lymph node contains lymphoid lobules (the functional units of the lymph node) surrounded by a lymph-filled subcapsular sinus enclosed by a capsule.

Lymph fluid enters the lymph node through afferent lymphatic vessels located at the apex or entrance of the lymph node.  The apex is on the opposite side of the lymph node to the hilum and the exiting efferent lymphatic vessel.

In terms of direction of lymph fluid flow, the afferent lymphatic vessel brings lymph fluid into the subcapsular sinus, which divides into many trabecular sinuses that penetrate the cortex, and lead into medullary sinuses which merge to funnel the lymph fluid out of the lymph node through the efferent lymphatic vessel.  As one can imagine, the initial subcapsular sinus is strategically divided into a complex network of transverse and medullary sinuses that surround each lymphoid lobule, so that lymph fluid is constantly flowing and coming into contact with the APCs, B cells and T cells.  Within the medulla, medullary cords are also made up of APCs and lymphocytes, and these cords of cells line medullary sinuses which carry lymph fluid and merge together into the efferent lymphatic vessel that exits the base of the lymph node at its hilum.

The lymph node is sometimes categorized as having cortex, paracortex, deep cortex and medulla regions.  The B cells clustered in follicles (nodules) in the cortex, once activated will proliferate to form germinal centres in the cortex, while T cells are housed and proliferate mainly within the paracortex and deep cortex regions closer to the medulla.  The structure and scaffolding of the lymph node is created by a reticular meshwork composed of fibroblastic reticular cells and reticular fibers, which help to create a multitude of transverse and medullary sinus channels that penetrate the lobules.  The sinuses are lined with lymphocytes, macrophages and other APCs. Macrophages (called sinus histiocytes) act like spiders within sinuses trapping and engulfing pathogens, cancerous cells, and debris.

As lymph fluid flows through the sinus of each region, (e.g., around the B cell germinal centres of the cortex and the T cell paracortex region), the lymph fluid is continually sampled and the interactions between the APCs and lymphocytes occur that can lead to activation of an immune response.  For example, activated B cells can produce precursor plasma cells that will migrate to the medullary cords where they mature and secrete antibodies into the medullary sinus lymph fluid. The medullary sinuses merge into the exiting efferent lymphatic vessel, which will bring the screened lymph fluid to the thoracic duct, so that lymph fluid will enter the subclavian vein.

Lymphatic vessels:  Lymph fluid is derived from interstitial fluid that is largely composed of plasma fluid that has leaked out of nearby capillaries.

Lymph fluid enters specialized lymphatic vessels, sometimes called blind-end lymphatic capillaries through flap-like minivalves.  These flaps in the thin endothelial walls of lymphatic capillaries allow fluid to enter but not exit.  As blood capillaries continually leak fluid into interstitial spaces while delivering oxygen and nutrients to tissue beds, it is important that this leaked fluid be returned to the vascular system in order to maintain blood volume.

The lymph fluid in lymphatic vessels is slowly pumped into the lymph nodes through lymphatic vessels with help from lymphatic vessel peristalsis.  Lymphatic vessels that direct fluid against gravity contain valves to prevent back-flow.

The lymphatic vessels that complete this task provide the added benefit of directing this fluid (now called lymph fluid) through lymph nodes that screen and remove any pathogens

MALT (Mucosa-Associated Lymphoid Tissue)

MALT is located just below the epithelial surfaces of many structures within the mucosa layer.  It contains lymphocytes (e.g., T cells, B cells, plasma cells) as well as phagocytes (e.g., dendritic cells and macrophages).  The M cells (microfold cells) within the intestinal epithelium provide an important role in immunosurveillance and it is thought that similar cells are found in other MALT as well.  M cells function by using phagocytosis, endocytosis or transcytosis to take up antigens and microorganisms from their apical (external) surfaces and deliver those antigens through their basal cell surface to the antigen-processing macrophage and dendritic cells (also known as antigen-presenting cells, APCs).  APCs and lymphocytes which upon processing and presenting these antigens are able to activate B and T cell immune responses against foreign antigens that are harmful (e.g., pathogenic).  For example, the B lymphocytes in this region produce immunoglobulin A (IgA) and immunoglobulin M (IgM) which help to defend against viral and bacterial infections.

MALT is found in various sites throughout the body, and includes the following subcategories:

• GALT:  The lamina propria is a thin layer of connective tissue below the epithelial cells of the mucosa of the gastrointestinal tract.  This is the location of the Gut-Associated Lymphatic Tissue (GALT), which due to its location is extensive in surface area and provides protection in an area which is vulnerable due to the thin mucosa.  The GALT includes the Peyer’s patches of the small intestines and the appendix.
• BALT and NALT:  Similar to GALT, the bronchus-associated lymphoid tissue (BALT) and nasal-associated lymphoid tissue (NALT) are part of the mucosal lining (just underneath the epithelium portion) and house lymphocytes.
• Mucosa-associated lymphoid tissue is also found in tear ducts, conjunctivae, arterial walls, and urogenital linings.
• A fun memory trick for MALTs are: NALT, BALT and GALT (No Bad Guys).

Tonsils

Tonsils are sometimes mistakenly called lymph nodes, when in fact they are known as lymphoid organs.  The tonsillar ring is an unlinked circular band of  mucosal-associated lymphoid tissues (MALT) that is arranged around the nasopharynx and oropharynx. Often simply known as a ring of 4 sets of tonsils that include: 1 pharyngeal (adenoid) tonsil located on the roof and posterior wall of the nasopharynx, 2 tubal tonsils where each auditory tube enters the nasopharynx, 2 palatine tonsils in the oropharynx, and the lingual tonsils at the back of the tongue.  The tonsils are similar in structure and function to the MALT regions described above housing M (microfold) cells, APCs, as well as B and T lymphocytes.  B cells produce antibodies such as IgAs in this zone.  Tonsillar crypts increase the surface area allowing for trapping of foreign material and antigen sampling.  It is most often the palatine tonsils that become inflamed during tonsillitis and are at times removed if they recurrently become swollen and obstructive. Tonsils tend to become more fibrous at age 8-10, and exhibit fatty atrophy during adulthood.  Anatomical obstructions can lead to disrupted breathing and some cases of sleep apnea.  Caution is required during tonsillectomies as the tonsils are well vascularized and close to branches of the trigeminal nerve (which provides sensory input from the face) and glossopharyngeal nerve (which provides sensory and hearing input from the ear as well as taste sensations from the posterior ⅓ of the tongue).  Repeat infections or other pathologies of the tonsils can lead to referred otalgia (outer ear pain) and dysgeusia (abnormal taste sensations).

Summary

• Overview of the lymphatic system
  • Lymphatic System Functions:
    • Play vital roles in non-specific (innate) and specific (adaptive) immune defense
    • Fights infection
    • Stimulates repair
  • Lymphatic System Composition: 1. Lymph fluid, 2. Lymph vessels, 3. Lymph nodes, 4. Lymphatic tissues, and 5. Lymphoid organs
    1. Lymph Fluid:  
       • Lymph fluid is derived from interstitial fluid (extracellular fluid in all tissue beds, which comes from plasma leaked from capillaries delivering nutrients, O₂, and H₂O)
    2. Lymph vessels:
       • Carry lymph fluid
       • Blind-end lymphatic capillaries have flap-like mini-valves that allow fluid to enter but not exit
       • Important in returning leaked plasma fluid to vascular system to maintain blood volume
       • Direction:
         • Lymph vessels carry lymph fluid from interstitial spaces (throughout the entire body) to lymph nodes where the lymph fluid is screened by B and T lymphocytes for pathogens.
         • Lymph nodes are interspersed along the length of lymph vessels
         • Lymph vessels carry lymph fluid towards the left and right thoracic ducts which join into left and right subclavian veins respectively allowing lymph fluid to enter into the bloodstream.
    3. Lymph nodes:
       • 450 lymph nodes throughout the body
       • Located where afferent lymphatic vessels converge
       • Axillary lymph nodes: located in armpit region
       • Cervical lymph nodes: located in head and neck region
       • Inguinal lymph nodes: located in groin region
       • Central Nervous System (CNS):
         • lacks lymph nodes,
         • contains blood-brain barrier
         • contains lymphatic drainage vessels to cervical lymph nodes
       • Lymph Node Structure: 
         • Lymphoid lobules: contain lymphocyte and antigen-presenting cells (APCs) (dendritic cells and macrophages)
         • Size: ~1.5cm
         • Shape: oval or kidney-like shape
       • Lymph Node Organization:
         • Subcapsular sinus:  lymph-fluid region surrounded by a capsule
         • Lymph Fluid Entry: afferent lymphatic vessels
         • Lymph Fluid Exit: through opposite hilum, efferent lymphatic vessel
         • Superficial to Deep:  Subcapsular sinus, Cortex, Paracortex, Deep Cortex, Medulla
           • The structure and scaffolding of the lymph node is created by a reticular meshwork of fibroblastic reticular cells and reticular fibers (long ropy proteins) which help to create sinus channels that are lined with lymphocytes, and macrophages that engulf and destroy pathogens.
    4. Lymphatic tissues: contain B and T lymphocytes that provide immune response
       • MALT (Mucosa-Associated Lymphoid Tissue):
       • Location: Just below the epithelial surfaces of many structures within the mucosa layer.
       • Cellular Components:
         1. Lymphocytes: T cells, B cells, plasma cells.
         2. Phagocytes: Dendritic cells and macrophages.
         3. M cells (microfold cells) in intestinal epithelium provide immunosurveillance
            • Function of M Cells:
            • Phagocytosis, endocytosis, transcytosis
            • Take up antigens and microorganisms from external surfaces
            • Deliver antigens to processing macrophage and dendritic cells (APCs) through basal cell surface.
            • APCs: Process and present antigens, activating B and T cell responses.
            • B Lymphocytes produce immunoglobulins (antibodies) to defend against viral and bacterial infections.
       • MALT subcategories:
         • GALT: Located in the GI tract (small intestines), includes Peyer’s patches
         • BALT and NALT : bronchial and nasal linings  lymphocytes
    5. Lymph organs:
       • Tonsils: Three pairs of tonsils in mouth for lymphocytes to screen for pathogens
         • Tonsils are unlinked circular band of mucosal-associated lymphoid tissues (MALT) in nasopharynx and oropharynx
         • Pharyngeal (adenoid) tonsils, Palatine tonsils and Lingual tonsils
       • Spleen: 
         • Size and Location:
           • Fist-sized, 7-14cm long, approximately 200g
           • Located directly under the left portion of the diaphragm
           • Lateral to the stomach and pancreas
           • Ventral to the adrenal gland and kidney at the level of the 10th rib
         • Spleen Functions:
           • Hematopoiesis during fetal life
           • RBC recycling
           • Stores RBCs, WBCs, and platelets to be released during emergencies
             • ~240mL of RBCs can be released to counter hypovolemia or hypoxia
           • Screens blood-borne pathogens
           • Houses T and B cells, providing adaptive (specific) immunity
           • Filters blood for pathogens and old/damaged/infected RBCs
         • Red and White Pulp Zones: described by coloured zone due to types of cells found in each
           • Red Pulp: 
             • Neutrophils, monocytes, macrophages, dendritic cells, and T cells
             • Function: Recycles old, infected and/or dysfunctional RBCs
           • White Pulp:
             • T and B lymphocytes
             • Function: Provides adaptive (specific) immune defense
• Two Primary Lymphoid Organs: bone marrow and thymus

• • Bone marrow:
    • Site of hematopoietic stem cell division
    • Gives rise to erythrocytes (RBCs), leukocytes (WBCs), and platelets (thrombocytes, essential for blood clotting)
    • RBCs: O₂ delivery, gas exchange in cardiovascular system
    • WBCs: Defense against pathogens and toxins, role in tissue repair.
      • Neutrophils, eosinophils, macrophages, monocytes = phagocytic cells that provide innate immunity and
      • Stimulate adaptive immunity through recruitment of B and T lymphocytes
      • B and T lymphocytes (B and T cells) = provide adaptive immunity through humeral (antibody) response and cytotoxic T cell responses respectively
      • Basophils and Mast cells = orchestrate inflammatory response through the release of vasoactive cytokines such as histamine
  • Thymus: contains cortex and medulla
    • Location of T lymphocyte maturation
    • Anterior mediastinum, behind sternum
    • 5cm at birth, increases to 50g during adolescence, atrophies in adulthood & replaced by fatty tissue
    • Outer cortex: Thymocytes (immature T cells) and specialized epithelial cells)
    • Thymosin hormones: immunoregulatory functions stimulate T cell maturation
• T cell Maturation Process:
  1. Immature T cells migrate from bone marrow to thymus cortex
  2. Specialized cortical epithelial cells play a role in positive selection:
  3. T cells challenged with self-antigens on MHC complexes
  4. T cells not binding at all to self-antigens undergo “death by neglect” apoptosis
  5. Passing T cells progress to medulla for negative selection:
  6. T cells binding too strongly to self-antigens undergo apoptosis
  7. T cells that pass positive and negative selection mature to express either CD4 or CD8 surface proteins
  8. Mature T cells that express CD8 proteins mature into Cytotoxic T cells and the CD8 proteins recognize MHC Class I complexes
  9. Mature T cells that express CD4 proteins mature into Helper T cells and the CD4 proteins recognize MHC Class II complexes
     • T cells passing positive and negative selection become self-tolerant
     • Surviving T cells are naive until challenged by a non-self (foreign) antigen