{"id":1816,"date":"2024-05-30T16:43:29","date_gmt":"2024-05-30T20:43:29","guid":{"rendered":"https:\/\/pressbooks.bccampus.ca\/pathophysiology\/?post_type=chapter&p=1816"},"modified":"2025-12-07T23:27:26","modified_gmt":"2025-12-08T04:27:26","slug":"overview-of-the-lymphatic-system","status":"web-only","type":"chapter","link":"https:\/\/pressbooks.bccampus.ca\/pathophysiology\/chapter\/overview-of-the-lymphatic-system\/","title":{"raw":"5p1 Overview of the Lymphatic System","rendered":"5p1 Overview of the Lymphatic System"},"content":{"raw":"

Overview of the Lymphatic System<\/h1>\r\nThe lymphatic system is composed of tissues and organs that play important roles in the body\u2019s immune system and ability to provide defense, fight infection and stimulate repair.\r\n\r\n \r\n\r\nBoth bone marrow and the thymus are considered to be the primary lymphoid organs.\r\n

Primary Lymphoid Organs<\/h1>\r\n

Bone Marrow<\/h3>\r\nBone 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.\u00a0 RBCs are involved in oxygen delivery and gas exchange within the cardiovascular system.\u00a0 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.\r\n\r\n \r\n

White Blood Cells<\/h4>\r\nWBCs include neutrophils, eosinophils, macrophages and monocytes that are important phagocytic cells that provide innate (non-specific) immune responses.\u00a0 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.\u00a0 The basophils and mast cells are WBCs that orchestrate the inflammatory response which can help to facilitate defense and tissue repair.\u00a0 basophils and mast cells that are important\r\n\r\n \r\n

Thymus<\/h3>\r\nThe thymus is considered to be a primary lymphoid organ as it is the location of T lymphocyte maturation.\u00a0 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.\u00a0 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.\u00a0 Thymosin hormones have immunoregulatory functions and stimulate the T cell maturation process.\r\n\r\nT 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.\u00a0 During this stage of maturation, T cells are challenged with self antigens displayed on MHC complexes.\u00a0 The T cells with T cell receptors (TCRs) that are not able to bind the MHC complexes at all, will go through \u201cdeath by neglect\u201d and go through apoptosis.\u00a0 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.\u00a0 T cells mature during this process and are only capable of expressing either CD4 or CD8 surface proteins. The T cells\u00a0 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. \u00a0 After passing both positive and negative selection tests, T cells are termed self-tolerant and self-MHC restricted.\u00a0 These surviving T cells are now termed naive, until they are challenged with a foreign antigen and launch a specific immune response.\r\n

Secondary Lymphoid Organs<\/h1>\r\nSecondary lymphoid organs include: the spleen, lymph nodes and mucosa-associated lymphoid tissues (MALT) and tonsils.\r\n\r\n \r\n

Spleen<\/h3>\r\nThe spleen is fist-sized, ranging from 7-14cm long, approximately 200g and is the largest of the secondary lymphoid organs.\u00a0 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.\r\n\r\nThe spleen has many functions.\u00a0 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.\u00a0 The spleen filters blood for pathogens and old\/damaged\/infected RBCs.\u00a0 The spleen stores RBCs, WBCs and platelets which can be released during times of emergency.\u00a0 For example, approximately 240mL of RBCs can be released in the event of hypovolemia and\/or hypoxia.\r\n\r\nThe spleen is often described by its red and white pulp zones which are coloured by the types of cells found in each.\u00a0 The red pulp contains neutrophils, monocytes, macrophages, dendritic cells, and T cells The red pulp zone recycles old, infected and\/or dysfunctional RBCs.\u00a0 The white pulp area contains T and B lymphocytes with each type in specific areas, and provides specific (adaptive) immune defense.\r\n

Lymph Nodes<\/h3>\r\nA 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.\r\n\r\n \r\n\r\nThere 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.\r\n\r\nEach lymph node contains lymphoid lobules full of lymphocytes and antigen-presenting cells (APCs), such as dendritic cells and macrophages.\u00a0 Lymph nodes are small, approximately 1.5cm in size, though ranging from 0.2-2.5cm, and oval or kidney-like in shape.\r\n\r\nEach lymph node contains lymphoid lobules (the functional units of the lymph node) surrounded by a lymph-filled subcapsular sinus enclosed by a capsule.\r\n\r\nLymph fluid enters the lymph node through afferent lymphatic vessels located at the apex or entrance of the lymph node.\u00a0 The apex is on the opposite side of the lymph node to the hilum and the exiting efferent lymphatic vessel.\r\n\r\nIn 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.\u00a0 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.\u00a0 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.\r\n\r\n \r\n\r\nThe lymph node is sometimes categorized as having cortex, paracortex, deep cortex and medulla regions.\u00a0 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.\u00a0 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.\u00a0 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.\r\n\r\nAs 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.\u00a0 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.\r\n\r\n \r\n\r\nLymphatic vessels:\u00a0 Lymph fluid is derived from interstitial fluid that is largely composed of plasma fluid that has leaked out of nearby capillaries.\r\n\r\nLymph fluid enters specialized lymphatic vessels, sometimes called blind-end lymphatic capillaries through flap-like minivalves.\u00a0 These flaps in the thin endothelial walls of lymphatic capillaries allow fluid to enter but not exit.\u00a0 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.\r\n\r\nThe lymph fluid in lymphatic vessels is slowly pumped into the lymph nodes through lymphatic vessels with help from lymphatic vessel peristalsis.\u00a0 Lymphatic vessels that direct fluid against gravity contain valves to prevent back-flow.\r\n\r\nThe 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\r\n\r\n \r\n

MALT (Mucosa-Associated Lymphoid Tissue)<\/h3>\r\nMALT is located just below the epithelial surfaces of many structures within the mucosa layer.\u00a0 It contains lymphocytes (e.g., T cells, B cells, plasma cells) as well as phagocytes (e.g., dendritic cells and macrophages).\u00a0 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.\u00a0 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).\u00a0 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).\u00a0 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.\r\n\r\n \r\n\r\nMALT is found in various sites throughout the body, and includes the following subcategories:\r\n