Copyright EMAP Publishing 2020 This article is not for distribution except for journal club use Clinical Practice Keywords Immunity/Anatomy/Stem cell production/Lymphatic system Systems of life This article has been Lymphatic system double-blind peer reviewed In this article... l How blood and immune cells are produced and developed by the lymphatic system l Clinical significance of the primary and secondary lymphoid organs l How the lymphatic system mounts an immune response and filters pathogens The lymphatic system 2: structure and function of the lymphoid organs Key points Authors Yamni Nigam is professor in biomedical science; John Knight is associate The lymphoid professor in biomedical science; both at the College of Human and Health Sciences, organs include the Swansea University. red bone marrow, thymus, spleen Abstract This article is the second in a six-part series about the lymphatic system. It and clusters of discusses the role of the lymphoid organs, which is to develop and provide immunity lymph nodes for the body. The primary lymphoid organs are the red bone marrow, in which blood and immune cells are produced, and the thymus, where T-lymphocytes mature. The Blood and immune lymph nodes and spleen are the major secondary lymphoid organs; they filter out cells are produced pathogens and maintain the population of mature lymphocytes. inside the red bone marrow, during a Citation Nigam Y, Knight J (2020) The lymphatic system 2: structure and function of process called the lymphoid organs. Nursing Times [online]; 116: 11, 44-48. haematopoiesis The thymus secretes his article discusses the major become either erythrocytes, leucocytes or hormones that are lymphoid organs and their role platelets. Stem cells rapidly multiply to essential for normal in developing and providing make billions of blood cells each day; this immune function Timmunity for the body. The lym- process is known as haematopoiesis and is and develops phoid organs include the red bone marrow, outlined in Fig 2. T-lymphocytes thymus, spleen and clusters of lymph To ensure there is a continuous produc- nodes (Fig 1). They have many functional tion and differentiation of blood cells to The spleen mounts roles in the body, most notably: replace those lost to function or age, hae- the immune l Production of blood cells, including red matopoietic stem cells are present through response and blood cells (erythrocytes), white blood adulthood. In the embryo, blood cells are removes micro- cells (leucocytes) and platelets initially made in the yolk sac but, as devel- organisms and (thrombocytes); opment of the embryo proceeds, this func- damaged red blood l Removal of damaged red blood cells; tion is taken over by the spleen, lymph cells from circulation l Maturation of immune cells; nodes and liver. Later in gestation, the l Trapping foreign material. bone marrow takes over most haematopoi- Lymph nodes are The red bone marrow and thymus are etic functions so that, at birth, the whole clustered considered to be primary lymphoid skeleton is filled with red bone marrow. throughout the body organs, because the majority of immune Red bone marrow produces all erythro- and filter pathogens cells originate in them. cytes, leucocytes and platelets. Haemat- from lymph, swelling opoietic stem cells in the bone marrow when mounting an The red bone marrow follow either the myeloid or lymphoid lin- immune response Bone marrow is a soft, gelatinous tissue eages to create distinct blood cells (Fig 2); present in the central cavity of long bones these include myeloid progenitor cells such as the femur and humerus. Blood (monocytes, macrophages, neutrophils, cells and immune cells arise from the bone basophils, eosinophils, erythrocytes, den- marrow; they develop from immature dritic cells and platelets), and lymphoid stem cells (haemocytoblasts), which progenitor cells (T-lymphocytes, B-lym- follow distinct developmental pathways to phocytes and natural killer cells). Nursing Times [online] November 2020 / Vol 116 Issue 11 44 www.nursingtimes.net Copyright EMAP Publishing 2020 This article is not for distribution except for journal club use Clinical Practice Systems of life Fig 1. The lymphoid organs in adults weighs around 40g, but in a middle-aged adult it may have shrunk sufficiently to be difficult to locate. By 20 years of age, the Primary lymphoid organs Secondary lymphoid organs thymus is 50% smaller than it was at birth, and by 60 years of age it has shrunk to a sixth of its original size (Bilder, 2016); this Neck (cervical) lymph nodes is called thymic involution Each of the two lobes of the thymus is surrounded by a capsule, within which are numerous small lobules – typically meas- Thymus uring 2-3mm in width – which are held Armpit (axillary) together by loose connective tissue. Each lymph nodes lobule consists of follicles that are com- posed of a framework of thyomsin- secreting epithelial cells and a population of T-lymphocytes; these cells are com- Spleen monly referred to as T-cells (the ‘T’ denotes their origin as mature cells from the thymus). Lobules have two distinct areas: l A dense outer cortex that is rich in Peyer’s patches actively dividing T-cells; l An inner medulla, which is much paler in colour and functions as an area of T-cell maturation. In addition to being a major lymphoid Red bone organ, the thymus is also recognised as marrow Groin (inguinal) part of the endocrine system because it lymph node secretes a family of hormones collectively referred to as thymosin; this is a group of several structurally related hormones secreted by the thymic epithelial cells. These hormones are essential for normal immune function and many members of the thymosin family are used therapeuti- cally to treat cancers, infections and dis- Some lymphoid cells (lymphocytes) Bone marrow diseases and transplants eases such as multiple sclerosis (Severa et begin life in the red bone marrow and Any disease or disorder that poses a threat al, 2019). become fully formed in the lymphatic to the bone marrow can affect many body organs, including the thymus, spleen and systems, especially if it prevents stem cells Role in T-cell maturation lymph nodes. As puberty is reached and from turning into essential cells. Those T-cells originate as haematopoietic stem growth slows down, physiological conver- known to damage the marrow’s productive cells from the red bone marrow (Fig 2). A sion occurs, changing red bone marrow to ability and destroy stem cells include: population of these haematopoietic stem yellow bone marrow. This entire process is l Leukaemia; cells infiltrate the thymus, dividing fur- completed by the age of 25 years, when red l Hodgkin’s lymphoma; ther within the cortical regions of the lob- bone marrow distribution shows its adult l Other lymphomas. ules then migrating into the medullary pattern in the bones. A growing number of diseases can be regions to mature into active T-cells; this The pattern is characterised by: treated with a bone marrow transplant or process of T-cell maturation is controlled l The presence of red bone marrow in the haematopoietic stem cell transfer; this is by the hormone thymosin. A proportion of axial skeleton (the vertebral bodies, often achieved by harvesting suitable these mature T-cells continually migrate sacral bone and medial parts of the hip donor stem cells from the posterior iliac from the thymus into the blood and other bones) and articular ends of the crests of the hip bone, where the concen- lymphoid organs (spleen and lymph humeral and femoral bones; tration of red bone marrow is highest. nodes), where they play a major role in the l The most distal parts of the skeleton body’s specific immune responses (which being filled with yellow bone marrow The thymus will be discussed in detail in part 3 of this only, which mainly acts as a store for The thymus gland is a bi-lobed, pinkish- series). The importance of these cells is fats, providing sustenance and grey organ located just above the heart in apparent in patients who have depleted maintaining the correct environment the mediastinum, where it rests below the T-cell populations, such as those infected for the bone to function. sternum (breastbone). Structurally, the with HIV. However, under particular conditions, thymus resembles a small bow tie, which One of the most important functions of such as severe blood loss or fever, the gradually atrophies (shrinks) with age. In the thymus is programming T-cells to rec- yellow marrow may revert back to red pre-pubescents, the thymus is a relatively ognise ‘self’ antigens through a process JENNIFER N.R. SMITH marrow (Malkiewicz and Dziedzic 2012). large and very active organ that, typically, called thymic education. This process Nursing Times [online] November 2020 / Vol 116 Issue 11 45 www.nursingtimes.net Copyright EMAP Publishing 2020 This article is not for distribution except for journal club use Clinical Practice Systems of life Fig 2. Haematopoiesis eventually becomes a tributary of the hepatic portal vein. The spleen is made up of two regions: Haematopoietic l Stroma – comprising the dense outer stem cells capsule with its trabeculae, some fibres and fibroblasts (cells that secrete connective tissue collagen); Eosinophils l Parenchyma – composed of two types Lymphoid Myeloid of intermingling tissue called white progenitor cells progenitor cells pulp and red pulp. White pulp is a mass of germinal centres Basophils of dividing B-lymphocytes (B-cells), sur- rounded by T-cells and accessory cells, including macrophages and dendritic cells; T-lymphocytes Platelets these cells are arranged as lymphatic nod- Neutrophils ules around branches of the splenic artery. Monocytes As blood flows into the spleen via the splenic artery, it enters smaller, central arteries of B- lymphocytes Erythrocytes the white pulp, eventually reaching the red pulp.