Copyright EMAP Publishing 2020 This article is not for distribution except for journal club use Clinical Practice Keywords Lymphatic system/ Immunity/Antigens/Lymphocytes Systems of life This article has been Lymphatic system double-blind peer reviewed In this article... ● The role of the lymphatic system in providing immune responses ● How different lymphocytes respond to pathogenic micro-organisms and toxins ● The role of the spleen in providing immunity The lymphatic system 3: its role in the immune system Key points Authors Yamni Nigam is professor in biomedical science; John Knight is associate The lymphatic professor in biomedical science; both at the College of Human and Health Sciences, system plays an Swansea University. important role in the immune system Abstract The lymphatic system plays an important role in providing immune responses to harmful micro-organisms and toxins that enter the body. This article, the third in a It produces a range six-part series on the system, discusses its main functions in providing immunity. of cells that detect and kill invading Citation Nigam Y, Knight J (2020) The lymphatic system 3: its role in the immune pathogenic system. Nursing Times [online]; 116: 12, 45-49. micro-organisms and toxins otential pathogens (micro- Immune cells of the Phagocytes can deal organisms capable of causing lymphatic system with many disease) are ubiquitous in the The immune system includes a vast range pathogens, but Penvironment and can enter the of distributed defence cells: these are the others need a more body via the skin (through direct contact, leucocytes (white blood cells). The more- specialised response particularly if the epidermis is injured basic innate immune leucocytes are the from lymphocytes through cuts, grazes or burns), the respira- first responders, responsible for imme- tory system (through inhalation), the gut diate and non-specific engagement with a Circulating lymph (through ingestion) and the genito- pathogen; they include phagocytes (cells transports some of urinary tract (through sex or the insertion capable of engulfing and absorbing bac- these cells around of invasive devices such as catheters). teria and other small cells and particles) the body to Although each of these sites is protected such as macrophages and dendritic cells, encounter pathogens by unique barriers and defences, some which encounter and indiscriminately ‘eat’ and toxins pathogens can breach these preliminary unwanted microbes or infected cells. defences and enter the body. If innate cells cannot deal with the path- Various ‘checkpoints’ The immune system comprises a range ogen, more-specialised cells known as in the lymphatic of cells – some basic and innate, others lymphocytes need to be ‘introduced’ to a system can raise a extremely specialised – to detect and pathogen to recognise it as a threat, before response when remove pathogens from the body. The lym- they can launch an attack on it. lymph containing phatic system works alongside the pathogens passes immune system to destroy unwanted Macrophages through them pathogens either locally and directly, or by These large-cell phagocytes are derived alerting the whole body to the infection from monocytes (large phagocytic white and helping to mount a wider systemic blood cells); they can be fixed in tissues or immune response. mobile in the blood. Macrophages are The first two articles in this series dis- capable of reeling in microbes with their cussed the role of lymph in supporting the cytoplasmic extensions (pseudopods) and cardiovascular system, and examined the engulfing them; they are tough cells that organs and tissues that make up the lym- survive well and can perform this function phatic system. This article focuses on the many times over. Macrophages are also immune function of the lymphatic system. able to trap antigens (small molecules Nursing Times [online] December 2020 / Vol 116 Issue 12 45 www.nursingtimes.net Copyright EMAP Publishing 2020 This article is not for distribution except for journal club use Clinical Practice Systems of life found on the surface of all cells) and pre- Fig 1. Major immune components of the lymphatic system sent them to other leucocytes of the immune system. Healthy cells in the body contain self- antigens, which act as important flags to Adenoids prevent the immune system from attacking Tonsils the body’s own cells. Phagocytes recognise pathogens and the toxins they may pro- duce as foreign bodies by the presence of their different (non-self ) antigens; they engulf and sequester (capture/trap) these foreign pathogens, which are then rapidly Thymus killed by intracellular digestion. Axillary lymph nodes Lymphocytes Bronchus-associated Lymphocytes are sentinel cells of adaptive lymphoid tissue immunity; they make up 20-30% of circu- lating leucocytes and include B-lympho- Spleen cytes and T-lymphocytes. Bone marrow harbours about 12% of the body’s lympho- cytes, whereas the spleen and lymph nodes Intestine contain approximately 55% of resident Peyer’s patches lymphocytes; the remainder are found in other lymphatic organs and tissues. Free Appendix lymphocytes in the blood amount to only about 2% of the lymphocyte population Bone marrow Inguinal lymph (Pabst, 2018). node B-lymphocytes B-lymphocyte cells (B-cells) are formed and mature in the bone marrow. Once released, they develop the ability to deter- mine which antigens they should react to factories – while a small proportion are other more-specialised lymphocytes to kill (immunocompetence) and which are retained as memory B-cells, able to quickly them. This ‘coating’ of foreign cells by harmless (self-tolerance). Mature B-cells mass produce the same antibodies again if antibody molecules is known as opsonisa- colonise secondary lymphoid organs such that particular antigen is encountered in tion and makes the pathogen more attrac- as lymph nodes or the spleen and: future. Plasma cells are capable of pro- tive to circulating phagocytes. ● Are responsible for humoral immunity ducing around 2,000 antibodies per Finally, when bound to their corre- (which acts via the humors – fluids second (Alberts et al, 2015). They are usu- sponding antigens, antibodies can activate such as blood or lymph); ally detectable in the humor (plasma) after a system of potent plasma enzymes of the ● Combat pathogens by producing and 4-7 days and float freely in blood and complement system. This group of 20 or so dispatching antibodies. lymph, binding to foreign antigens on the proteins, once activated, forms a protein Collectively called immunoglobulins surface of the pathogen or to the toxin that conglomerate – termed a membrane attack (Ig), antibodies are among the most abun- triggered their formation. complex (MAC) – which attacks and rup- dant protein components in the blood and tures pathogens’ membranes leading to cell an important part of the immune system The role of antibodies. Although they cannot lysis and death. The complement system (Alberts et al, 2015). As a naïve B-cell directly destroy antigens or kill pathogens can be activated by antigen-antibody com- becomes fully mature, it can display thou- themselves, the action of antibodies can: plexes or recognition of bacteria. sands of membrane-bound antibodies on ● Help to inhibit micro-organisms; its surface, and each B-cell has its own ● Highlight them for detection and T-lymphocytes unique set of these ready to identify and attack by other immune cells. Pathogens are not always found in fluids; bind to a particular antigen. If a random First, as antibodies bind to the foreign many become intracellular (invade the encounter with a potentially pathogenic pathogens, they neutralise them by physi- cells) where antibodies cannot reach. For- foreign antigen results in binding and trig- cally blocking binding sites on the path- tunately, another branch of adaptive gering of any of these membrane anti- ogen so it cannot attach to tissue cells and immunity can provide more direct cell-to- bodies, it activates the B-cell. cause disease. cell combat. This is cell-mediated immu- The B-cell rapidly clones itself, forming Second, antibodies cause agglutination nity and is facilitated by the T-lympho- masses of B-cells, all with the same of pathogens as they can bind to more than cytes (T-cells), which are produced in the instructions for producing the antibody one antigen simultaneously. Agglutinated bone marrow and mature in the thymus designed to fight that particular antigen. pathogens clump and cannot move around gland (see part 2). The majority of these cloned cells become as easily, so it is easier for macrophages to There are a few different types of JENNIFER N.R. SMITH B-plasma cells – large antibody-producing detect and phagocytose them, and for T-cells, but the two main ones are: Nursing Times [online] December 2020 / Vol 116 Issue 12 46 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. Immune reactions inside a lymph node discussed the importance of tissue drainage and the transport of lymph back into the circulatory system to ensure Helper T-cell homoeostasis. While lymph is circulating contacting Activated helper around the body, it passes through various displayed antigen T-cell interacts with ‘checkpoint’ sites of the lymphatic system; B-cell and releases these sites include lymph nodes, the cytokines, which spleen and various types of mucosa-asso- activate the B-cell ciated lymphoid tissue (MALT) (Fig 1). The lymph nodes, in particular, play a major role in trapping foreign material. Displayed antigen Immunity at the lymph nodes Macrophage Approximately 600-700 lymph nodes are displaying antigen situated in clusters around the body in Cytokines lymphatic vessels; they range from about stimulate 1-2mm to 2cm in size and are often pal- B-cell to pable in the neck, armpit and groin. proliferate These tightly packed balls of lymphoid Antigen receptor cells and protein primarily act to: ● Monitor lymph arriving at each node B-cell combining for pathogens that may have entered with antigen Activated B-cell the system; ● Attempt to eliminate them before they can cause any damage to the body.