Secondary Lymphoid Structures PDF

21.4 Secondary Lymphoid Structures Structures that house both lymphocytes and other immune cells are called secondary lymphoid structures. These structures are composed of lymphoid cells that are enmeshed within a reticular connective tissue matrix (see section 5.2d). Secondary lymphoid structures a...

21.4 Secondary Lymphoid Structures Structures that house both lymphocytes and other immune cells are called secondary lymphoid structures. These structures are composed of lymphoid cells that are enmeshed within a reticular connective tissue matrix (see section 5.2d). Secondary lymphoid structures are organized into both lymphoid organs and aggregates of lymphoid nodules. They are differentiated by the presence or absence of a capsule composed of dense irregular connective tissue that encloses the lymphoid structure. A complete capsule is present in lymphoid organs, which include lymph nodes and the spleen. A capsule is either incomplete or absent in other lymphoid structures, which include tonsils, lymphoid nodules, and MALT. 21.4a Lymph Nodes LEARNING OBJECTIVE 9. Describe both the structure and function of lymph nodes. Lymph nodes are small, round or oval, encapsulated structures located along the pathways of lymphatic vessels, where they serve as the main lymphoid organ. Lymph nodes function in the filtering of lymph and removal of unwanted substances. Lymph nodes vary in both their size (from 0.1 to 2.5 centimeters) and their number (estimated between 500 and 700 throughout the entire body). Lymph nodes are located both superficially and deep within the body and typically occur in clusters that receive lymph from selected body regions. Some examples of clustered lymph nodes include the cervical lymph nodes that receive lymph from the head and neck; the axillary lymph nodes in the armpit that receive lymph from the breast, axilla, and upper limb; and inguinal lymph nodes in the groin that receive lymph from the lower limb and pelvis (see figure 21.1). In addition to clusters, lymph nodes are individually distributed throughout the body. Page 842 Numerous afferent lymphatic vessels transport lymph into a lymph node ( figure 21.6). There is typically only one efferent lymphatic vessel, which originates at the involuted portion of the lymph node called the hilum (hī′lŭm), or hilus. Lymph is drained via the efferent lymphatic vessel from this region of the lymph node. Figure 21.6 Lymph Nodes. (a) Lymph nodes are small, encapsulated organs that filter the lymph that is transported to them by lymphatic vessels. Green arrows indicate the direction of lymph flow into and out of the lymph node. (b) The arrangement of regions within the cortex and medulla of a lymph node. (c) A micrograph of a lymph node shows the cortex and medulla. (c) ©Ed Reschke/Getty Images APR Module 10: Lymphatic: Histology: Lymph Node: LM Low Magnification: Capsule of Lymph Node The capsule of a lymph node is composed of dense irregular connective tissue that encapsulates the node. The capsule has numerous internal extensions called trabeculae, which both subdivide the node into compartments and provide support for blood vessels and nerves that are within the lymph node. The lymph node regions internal to the capsule are subdivided into an outer cortex and an inner medulla. The cortex is composed in part of multiple lymphoid nodules. Each lymphoid nodule within the lymph node is composed of reticular fibers (see section 5.2a) which support an inner germinal center that houses both proliferating B-lymphocytes and some macrophages. The germinal center is surrounded by an outer region called a mantle zone, which contains T-lymphocytes, macrophages, and dendritic cells (described in section 22.2a). The medulla differs from the cortex because it has strands of connective tissue fibers that support B-lymphocytes, T-lymphocytes, and macrophages. These structures are called medullary cords. Both the cortex and medulla of a lymph node also contain small, open channels called cortical sinuses and medullary sinuses, respectively. These spaces are lined with macrophages. Lymph Flow Through Lymph Nodes Lymph enters a lymph node through numerous afferent lymphatic vessels, then flows through the lymph node sinuses, before typically draining through one efferent lymphatic vessel. Numerous afferent lymphatic vessels and generally one efferent lymphatic vessel are significant in the normal function of a lymph node. Consider that in this anatomic arrangement the collective diameter of the “inflow pipes” (i.e., afferent lymphatic vessels) is larger than the diameter of the “outflow pipe” (i.e., efferent lymphatic vessel). This creates a higher fluid pressure in the intervening structures (i.e., the lymph node sinuses), which helps to force the lymph through the lymph node. Lymph is continuously monitored for the presence of foreign or pathogenic substances as it passes through nodes. Macrophages residing in the lymph node remove foreign debris from the lymph by phagocytosis (see section 22.3c). Lymph then exits the lymph node through an efferent lymphatic vessel. Recall that lymph nodes are often found in clusters, so after one lymph node receives and filters lymph, the lymph is then transported to another lymph node in the cluster, then to another, and so on. Thus, lymph is repeatedly screened for unwanted substances. Page 843 INTEGRATE CLINICAL VIEW 21.3 Lymphoma A lymphoma (lim-fō΄mă; oma = tumor) is a malignant neoplasm (cancer) that develops within lymphoid structures. These tumors develop most often from abnormal B-lymphocytes, and less commonly from abnormal T‑lymphocytes. Usually (but not always), a lymphoma presents as a nontender, enlarged lymph node, often in the neck or axillary region. Some patients have no further symptoms, whereas others may experience night sweats, fever, and unexplained weight loss in addition to the nodal enlargement. Lymphomas are grouped into two categories: Hodgkin lymphoma and non-Hodgkin lymphoma. Hodgkin lymphoma (or Hodgkin disease) is characterized by the presence of the Reed-Sternberg cell, a large cell whose two nuclei resemble owl eyes, surrounded by lymphocytes within the affected lymph node. Hodgkin lymphoma affects young adults (ages 16–35) and people over 60. It arises in a lymph node and then spreads to other nearby lymph nodes. If caught early, Hodgkin lymphoma can be treated and cured by excision of the tumor, followed by radiation, chemotherapy, or both. Non-Hodgkin lymphomas are much more common than Hodgkin lymphomas. Some kinds of non-Hodgkin lymphoma are aggressive and often fatal, whereas others are slow-growing and more responsive to treatment. Treatment depends upon the type of non-Hodgkin lymphoma, the extent of its spread at the time of discovery, and the rate of progression of the malignancy. Reed-Sternberg cell, a characteristic of Hodgkin lymphoma. NYU Franklin Research Fund/Phototake INTEGRATE CONCEPT CONNECTION Dendritic cells are specialized phagocytic cells that are formed in bone marrow and housed in both epithelial and connective tissue of the skin (see sections 6.1a and 6.1b) and mucosal membranes. Those cells within the epidermis of the skin are specifically called epidermal dendritic cells (see section 6.1a). Dendritic cells migrate from the skin and mucosal membranes to a lymph node following phagocytosis of foreign substances (see section 22.2a). NK (natural killer) cells are a type of lymphocyte formed in the lymphoid line (see section 18.3a). These innate immune cells (see section 22.3c) differentiate and mature within the thymus and secondary lymphoid structures before entering and circulating in the blood. Lymphocytes housed within the lymph node also come into contact with foreign substances. An immune response may be initiated after this contact, during which lymphocytes undergo cellular division, especially in the germinal centers. Some of these new lymphocytes remain within the lymph node, whereas others are transported within the lymph and then enter the blood, to ultimately reach areas of infections (see section 22.6). When a person has an infection, often some lymph nodes are swollen and tender to the touch. This is a condition erroneously termed swollen glands. These enlarged nodes are a sign that lymphocytes are proliferating and attempting to fight an infection. Swollen superficial lymph nodes, such as those in the neck and axilla, can generally be palpated (felt). WHAT DID YOU LEARN? 8 How does lymph flow through a lymph node, and how is it monitored by macrophages and lymphocytes? 21.4b Spleen LEARNING OBJECTIVES 10. Describe the location, structure, and function of the spleen. 11. Distinguish between white pulp and red pulp. The spleen (splēn) is the largest lymphoid organ in the human body. It is located in the left upper quadrant of the abdomen, inferior to the diaphragm and adjacent to ribs 9–11 ( figure 21.7a). This deep red organ lies lateral to the left kidney and posterolateral to the stomach. The spleen can vary considerably in size and weight, but typically is about 12 centimeters (5 inches) long and 7 centimeters (3 inches) wide. Figure 21.7 Spleen. (a) A cadaver photo shows the position of the spleen within the body. The pancreas has been moved inferiorly in this photo to show more clearly the splenic vessels. (b) A second photo shows the medial surface of the spleen, where the splenic artery and splenic vein extend from the hilum. A diagram (c) and micrograph (d) of spleen histology depict the microscopic arrangement of red pulp and white pulp. (a, b) ©McGraw-Hill Education/Christine Eckel; (d) ©McGraw-Hill Education/Al Telser APR Module 10: Lymphatic: Histology: Spleen: Capsule of Spleen The spleen’s posterolateral aspect (called the diaphragmatic surface) is convex and rounded; the concave anteromedial border (the visceral surface) contains the hilum (or hilus), where blood vessels and nerves connect to the spleen ( figure 21.7b). The splenic (splen′ik) artery delivers blood to the spleen, whereas blood is drained by the splenic vein. The spleen is surrounded by a connective tissue capsule from which trabeculae extend into the organ ( figure 21.7c, d). The spleen lacks a cortex and medulla. Rather, the trabeculae subdivide the spleen into regions of white pulp and red pulp. White pulp consists of spherical clusters of T-lymphocytes, B-lymphocytes, and macrophages, which surround a central artery. The remaining splenic tissue, called red pulp, contains erythrocytes, platelets, macrophages, and B-lymphocytes. The cells in red pulp are housed in reticular connective tissue (see section 5.2d) and form structures called splenic cords (cords of Bilroth). Splenic sinusoids are associated with red pulp. (Recall from section 20.1c that sinusoids are very permeable capillaries that have an incomplete basement membrane, so formed elements (erythrocytes, leukocytes, and platelets) can easily enter and exit across the vessel wall.) The blood within sinusoids drains into small venules that ultimately lead into the splenic vein. Red pulp of the spleen serves as a blood reservoir, including a storage site for both erythrocytes and platelets (about 30% of all platelets are stored in the spleen). In situations where more erythrocytes and platelets are needed, such as during hemorrhage, these stored formed elements reenter the blood (see sections 18.3b and 18.3d). Page 844 WHAT DO YOU THINK? 2 Predict the consequences to the spleen during an accident if an individual wears his or her seat belt more superiorly across the abdomen instead of more inferiorly across the pelvis. Monitoring Blood as It Flows Through the Spleen The spleen functions to filter blood (not lymph, which is filtered by lymph nodes). As blood enters the spleen and flows through the central arteries, the white pulp monitors the blood for foreign materials, bacteria, and other potentially harmful substances. After passing through a central artery, blood is transported through sinusoids of red pulp. As blood moves through the sinusoids, macrophages lining the sinusoids phagocytize bacteria and foreign debris from the blood, as well as both old and defective erythrocytes and platelets. Thus, the general flow of blood through the spleen is the splenic artery, the central artery (of white pulp), the splenic sinusoid (of red pulp), venules (that drain sinusoids), and ultimately the splenic vein ( figure 21.7). In summary, the spleen serves several functions, including: phagocytosis of bacteria and other foreign materials in the blood as part of the body’s defense (red and white pulp); phagocytosis of old, defective erythrocytes and platelets from circulating blood (red pulp); and a role as a blood reservoir and storage site for both erythrocytes and platelets (red pulp). INTEGRATE CLINICAL VIEW 21.4 Splenectomy Splenectomy (splē-nek΄tō-mē) is surgical removal of the spleen. This procedure may be performed for several reasons, including a severe splenic infection or cyst within the spleen, Hodgkin lymphoma or other types of cancers, and certain blood disorders (e.g., sickle-cell disease). The most common reason, however, is a ruptured spleen resulting from abdominal injury. If the spleen is removed, other lymphoid structures may take over many of the spleen’s functions, but the person may be more prone to contracting a serious or life-threatening infection. For this reason, individuals who have had their spleens removed are encouraged to receive vaccines against the flu and pneumonia and are placed on antibiotic therapy for an extended period of time, sometimes for the rest of their lives. During fetal development through the fifth month, the spleen also engages in the production of formed elements, a function performed after birth by the red bone marrow (see section 18.3a). This function remains latent in the spleen and may be reactivated under certain conditions (e.g., some hematologic disorders). This process of reactivation is called extramedullary hematopoiesis (or hemopoiesis). Page 845 WHAT DID YOU LEARN? 9 What are the general functions of the spleen? Indicate whether red pulp or white pulp is responsible for each function. 10 Which lymphoid structures filter lymph? Which filters blood? 21.4c Tonsils LEARNING OBJECTIVE 12. Identify the main groups of tonsils and their location and function. Tonsils (ton′sillz; tonsilla = a stake) are secondary lymphoid structures that are not completely surrounded by a connective tissue capsule. They are found in the pharynx (throat) and oral cavity, and help protect against foreign substances that are either inhaled or ingested. Tonsils have invaginated outer edges called tonsillar crypts to increase the tonsil’s surface area to help trap material. Lymphoid nodules, some containing germinal centers, are housed within the tonsils. Tonsils are named according to their location in the nasopharynx or oral cavity. The pharyngeal tonsil is found in the posterior wall of the nasopharynx; when this tonsil becomes enlarged, it is called adenoids (ad′ĕ-noydz; aden = gland). Palatine tonsils are in the posterolateral region of the oral cavity, and lingual tonsils are along the posterior one-third of the tongue ( figure 21.8). Figure 21.8 Tonsils. (a) Tonsils reside in the wall of the oral cavity and pharynx. Micrographs show (b) tonsillar crypts that help trap foreign substances and (c) the germinal centers within the lymphoid nodules of the tonsils. (b) ©Dr. Thomas Caceci, Virginia-Maryland Regional College of Veterinary Medicine; (c) ©McGraw-Hill Education/Alvin Telser APR Module 10: Lymphatic: Dissection: Tonsils: Lateral: Pharyngeal Tonsil WHAT DID YOU LEARN? 11 What are the main groups of tonsils and their function? 21.4d Lymphoid Nodules and MALT LEARNING OBJECTIVES 13. Describe the composition of individual lymphoid nodules. 14. Compare the locations of MALT and Peyer patches. Lymphoid nodules and MALT make up the last category of secondary lymphoid structures. These relatively small masses of lymphoid structures are located throughout the body. Lymphoid Nodules Lymphoid nodules (nod′ūlz), or lymphoid follicles, are small, oval clusters of immune cells (e.g., B-lymphocytes, T-lymphocytes, macrophages) with some extracellular matrix that are not completely surrounded by a connective tissue capsule. Scattered lymphoid nodules are referred to as diffuse lymphoid tissue. This tissue can be found in every body organ and within the wall of the appendix, where it helps to defend against infections in these structures. However, in some areas of the body, many lymphoid nodules group together to form larger structures, such as MALT. INTEGRATE CLINICAL VIEW 21.5 Tonsillitis and Tonsillectomy The tonsils help to protect the pharynx from infection. They frequently become inflamed and infected, a condition called acute tonsillitis (ton′si-lī′tis). The palatine tonsils are the tonsils that are most commonly affected. The tonsils redden and enlarge; in severe cases, they may partially obstruct the pharynx and cause respiratory distress. Tonsils may be infected by viruses such as adenoviruses or bacteria such as Streptococcus. Streptococcal tonsillitis often results in very red tonsils that have whitish areas (called tonsillar exudate). The symptoms of tonsillitis include fever, chills, sore throat, and difficulty swallowing. Persistent or recurrent infections may result in permanent enlargement of the tonsils and development of a condition that is called chronic tonsillitis. If medical treatment does not help the chronic tonsillitis, surgical removal of the tonsils (tonsillectomy) may be indicated. Medical guidelines suggest performing a tonsillectomy only if the person has had 7 throat infections (e.g., tonsillitis, strep throat) in 1 year, 5 infections per year for 2 years, or 3 infections per year for 3 years. Research indicates that tonsillectomy does not significantly affect the body’s response to new infections. Acute streptococcal tonsillitis. Notice the abundant tonsilar exudate. ©Dr. P. Marazzi/Science Source MALT MALT (mucosa-associated lymphoid tissue) is located in the lamina propria of the mucosa (see section 5.5b) of the gastrointestinal, respiratory, urinary, and reproductive tracts. The immune cells in the MALT help defend against foreign substances that come in contact with mucosal membranes. MALT is very prominent in the mucosa of the small intestine, primarily in the ileum. There, collections of lymphoid nodules called Peyer patches (previously known as Peyer’s patches) can become quite large and bulge into the gastrointestinal tract lumen (see figure 26.16). Figure 21.9 is a visual representation of the role of the lymphatic system, which assists the cardiovascular system in maintaining fluid balance by returning excess fluid from the interstitial space to the blood, and assists the immune system by participating in defending the body against potentially harmful substances. Page 846 INTEGRATE CONCEPT OVERVIEW Figure 21.9 Relationship of the Lymphatic System to Both the Cardiovascular System and Immune System. The lymphatic system assists both (a) the cardiovascular system by returning fluid from the interstitial space back into the blood to help maintain fluid balance, blood volume, and blood pressure and (b) the immune system in the body’s defense. WHAT DID YOU LEARN? 12 What is the function of MALT in the mucosal linings of the gastrointestinal, respiratory, urinary, and reproductive tracts?