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Transcript
CHAPTER 24 Answers to “What Did You Learn?” 1. An immune response is a systematic defense against antigens (abnormal substances to the body) by lymphatic cells. Some lymphatic cells produce soluble proteins, antibodies that bind to and destroy the antigen while other lymphatic cells attack and destroy the antigens directly. 2. Lymph is the combination of interstitial fluid, solutes, and sometimes foreign material that has entered the lymphatic capillaries. 3. The smallest lymph vessels are lymphatic capillaries. Lymphatic capillaries are closed-ended tubes that are interspersed among most blood capillary networks except those within the red bone marrow and central nervous system. A lymphatic capillary is similar to a blood capillary in that its wall is an endothelium, however, it is larger in diameter than blood capillaries, lacks a basement membrane, and has overlapping endothelial cells. Lymphatic capillaries have one-way flaps that allow lymph to enter but not escape, and these capillaries eventually drain into lymphatic vessels. 4. The right lymphatic duct receives lymph from the lymphatic trunks that drain the right side of the head and neck, right upper limb, and right side of the thorax. 5. The types of lymphatic cells are macrophages, some epithelial cells (called nurse cells), dendritic cells, and lymphocytes. 6. T-lymphocyte types include helper T-lymphocytes that are needed to begin an effective defense against antigens, initiate and then oversee the immune response; cytotoxic T-lymphocytes, which come into direct contact with infected or foreign 24-1 cells and kill them; memory T-lymphocytes arise from T-lymphocytes that have encountered a foreign antigen, patrol the body and if they encounter the same antigen again, they mount an even faster immune response; and regulatory Tlymphocytes appear to turn off the immune response once it has been activated to help regulate its performance. B-lymphocyte types include activated lymphocytes that produce antibodies, and memory B-lymphocytes that mount an even faster and more powerful immune response at the next encounter with the antigen. 7. Lymphopoiesis is the process of lymphocyte development and maturation. All lymphocytes originate from a hemopoietic stem cell found in the red bone marrow. T-lymphocytes mature in the thymus while B-lymphocytes mature in the red bone marrow. 8. MALT (mucosa–associated lymphatic tissue) is composed of large collections of lymphatic nodules located in the mucosa and found in the GI tract, respiratory tract, and genitourinary tract. 9. The thymus functions as a site for T-lymphocyte maturation and differentiation. T-lymphocytes within the thymus do not participate in the immune response and are protected from antigens by a well-formed blood–thymus barrier around the blood vessels in the cortex. 10. Lymph nodes are small, round or ovoid structures located along the pathways of lymph vessels. Each lymph node is surrounded by a tough connective tissue capsule with internal extensions called trabeculae. The tissue deep to the lymph node capsule is subdivided into an outer cortex and an inner medulla. Lymph nodes function to filter lymph and mount an immune response. 24-2 11. The white pulp is associated with the arterial supply of the spleen and consists of circular clusters of lymphatic tissue (T-lymphocytes, B-lymphocytes, and macrophages). The red pulp is associated with the venous supply of the spleen. It consists of splenic cords and splenic sinusoids. The white pulp mounts an immune response when needed, while the red pulp serves as a reservoir for blood as well as phagocytize aged erythrocytes and platelets. 12. The reduced number of lymphocytes in an elderly person means that the body’s ability to acquire immunity and resist infection decreases. In addition, the lymphocytes that remain appear less able to target malignant cells, which may be one reason why the elderly are more prone to developing cancers. 13. The spleen begins to form during the fifth week of embryonic development. Answers to “Content Review” 1. The functions of the lymphatic system include reabsorbing and transporting excess interstitial fluid back to the bloodstream, transporting dietary lipids and lipid-soluble vitamins from the intestine to the bloodstream, storing and assisting in lymphocyte development, and generating an immune response. 2. Lymph is interstitial fluid that is transported in the lymph vessels. Once lymph enters the lymphatic capillaries, it is transported to and from lymph nodes via lymphatic vessels, lymphatic trunks, and finally lymphatic ducts before being emptied into the venous circulation. Each lymphatic duct drains at the junction of the internal jugular and subclavian veins. 3. The thoracic duct drains most regions of the body, including the left side of the 24-3 head and neck, left upper limb, left thorax, and all body regions inferior to the diaphragm, including the right lower limb and right side of the abdomen. 4. Macrophages are monocytes that have migrated from the bloodstream into lymphatic structures. They are responsible for phagocytosis of foreign substances and may present antigens to the other lymphatic cells. Special epithelial cells (also called nurse cells) are found in the thymus and secrete thymic hormones. Dendritic cells are found in lymphatic nodules; they internalize antigens from the lymph and present them to other lymphatic cells. These cells are the main antigen-presenting cell of the immune system. Lymphocytes are the most abundant cells in the lymphatic system. There are different types of lymphocytes: T-lymphocytes, B-lymphocytes, and natural killer (NK) cells. All migrate through the lymphatic system and search for antigens. (1) T-lymphocytes: helper T-lymphocytes initiate and manage the immune response; cytotoxic Tlymphocytes kill by secreting substances into foreign or abnormal cells; memory T-lymphocytes mount an even faster immune response at the next encounter with the antigen; and regulatory T-lymphocytes turn off the immune response. (2) Blymphocyte types include plasma cells that produce antibodies, and memory Blymphocytes that mount an even faster immune response at the next encounter with the antigen. Natural killer (NK) cells are large, granular lymphocytes that kill infected or cancerous cells. 5. Helper T-lymphocytes recognize an antigen and then stimulate the production of cytotoxic T-lymphocytes and B-lymphocytes. The cytotoxic T-lymphocytes will mount a cellular attack against foreign cells while B-lymphocytes will form 24-4 plasma cells that produce antibodies against the antigens. Both types of lymphocytes produce memory cells that will recognize the antigen and help mount a faster and more powerful immune response if the antigen reappears. 6. Lymphatic nodules are ovoid clusters of lymphatic cells with some extracellular connective tissue matrix that are not surrounded by a connective tissue capsule. The pale center of a lymphatic nodule is called the germinal center, which contains proliferating B-lymphocytes and some macrophages. T-lymphocytes are located outside the germinal center. Individually, a lymphatic nodule is small. However, sometimes lymphatic nodules will group together, forming larger structures such as mucosa-associated lymphatic tissue (MALT) or tonsils. 7. In infants and young children, the thymus is quite large and extends into the superior mediastinum. The thymus continues to grow until puberty, when the organ reaches a maximum weight of 30–50 grams. During this time, the thymus is the site for maturing and differentiating T-lymphocytes. By adulthood, the Tlymphocyte maturation process is complete and differentiated T-lymphocytes are produced only by mitosis of existing T-lymphocytes. Because the adult thymus is no longer an active T-lymphocyte maturation site, the thymic tissue regresses and the functional thymus is replaced by adipose connective tissue. In adults, the thymus atrophies. 8. Each lymph node is surrounded by a tough connective tissue capsule and has internal extensions called trabeculae projecting into the node, subdividing it into compartments. Deep to the lymph node capsule there is an outer cortex and an inner medulla. The cortex consists of lymphatic nodules and cortical sinuses 24-5 (lymphatic sinuses found in the cortex). The medulla contains strands of lymphatic tissue (consisting primarily of B-lymphocytes and macrophages) called medullary cords and medullary sinuses (lymphatic sinuses found in the medulla). The function of the lymph node is to filter the lymph, and mount an immune response if antigens are found in the lymph. 9. The arterial supply to the spleen is associated with the white pulp and consists of circular clusters of T-lymphocytes, B-lymphocytes, and macrophages. The white pulp detects antigens in the blood and mounts an immune response if antigens are found. The red pulp is associated with the venous drainage of the spleen. It is formed by splenic cords and splenic sinusoids. The red pulp is a reservoir of erythrocytes and the macrophages here remove aged or damaged erythrocytes. 10. The lymph vessels and lymph nodes first start to form during week six of embryonic development, when the primary lymph sacs form. Paired lymphatic vessels connect the lymph sacs by the ninth week of development. Eventually, a single thoracic duct forms, and the right lymphatic duct develops as well, and new lymph vessels continue to develop during and after the embryonic period. During the fetal period, the lymph sacs develop into rounded lymph nodes. 24-6