Detailed Chapter Summary Chapter 22, The Lymphatic System and Immunity
22.1 Lymph and Lymphatic Vessels (p. 610) The lymphatic system consists of lymph and lymphatic vessels, tissues, and organs.
The system serves to recover tissue fluid and maintain fluid balance; provide immune cells and monitor the body fluids for foreign matter; and transport dietary lipids from the small intestine to the blood.
Lymph is usually a colorless liquid similar to blood plasma, but intestinal lymph is milky when absorbing digested lipids. Lymph contains lymphocytes, macrophages, and hormones, and may contain metastasizing cancer cells, cellular debris, bacteria, and viruses.
Lymph originates in blind lymphatic capillaries that pick up tissue fluid throughout the body. The endothelial cells of lymphatic capillaries have large gaps between them that permit cells and other large particles to enter the lymph stream.
Lymphatic capillaries converge to form larger collecting vessels with a histology similar to that of veins. Lymph nodes lie at irregular intervals along the collecting vessels and filter the lymph on its way back to the blood.
Collecting vessels converge to form six lymphatic trunks that drain specific regions of the body. The lymphatic trunks then converge to form two collecting ducts—the right lymphatic duct and thoracic duct—which empty lymph into the subclavian veins.
There is no heartlike pump to move the lymph; lymph flows under forces similar to some of those that drive venous blood flow, and like some veins, lymphatic vessels have valves to ensure a one-way flow despite the low pressure.
22.2 Lymphatic Cells, Tissues, and Organs (p. 614) The cells of lymphatic tissue are natural killer (NK) cells, T lymphocytes, B lymphocytes, macrophages, dendritic cells, and reticular cells.
NK cells provide a nonspecific defense against bacteria, viruses, tissue transplants, and cancer. Their constant patrolling of the body for infected, cancerous, or defective cells is called immune surveillance.
T lymphocytes are named for their development in the thymus. The types of T cells are cytotoxic T (TC) cells, which directly attack and destroy enemy cells; helper T (TH) cells, which stimulate other lymphocytes as well as function in inflammation; regulatory T (TR) cells, which inhibit other T cells and keep immune responses from running out of control; and memory T (Tm) cells, which provide lasting immunity after the initial exposure to an antigen.
B lymphocytes attain maturity in the bone marrow. Upon exposure to an antigen, some of them develop into plasma cells, which secrete protective antibodies, and others into memory B cells.
Macrophages are large, highly phagocytic cells that develop from monocytes that have emigrated from the bloodstream into the tissues. They engulf and destroy foreign matter and dead host cells, and act as antigen-presenting cells (APCs) to activate immune responses. There are several kinds of macrophages, including microglia and alveolar and hepatic macrophages.
Dendritic cells are stationary macrophages of the skin, mucous membranes, and lymphatic organs. They engulf foreign matter by receptor-mediated endocytosis rather than phagocytosis, but otherwise act like any other macrophage.
Reticular cells are branched stationary cells that make up part of the stroma of lymphatic organs and act as APCs in the thymus.
Diffuse lymphatic tissue is an aggregation of lymphatic cells in the walls of other organs, especially in the mucous membranes of the respiratory, digestive, urinary, and reproductive tracts—where it is called mucosa-associated lymphatic tissue (MALT). In some places, lymphocytes and macrophages form dense masses called lymphatic nodules, such as the Peyer patches of the ileum.
Lymphatic organs have well-defined anatomical locations and have a fibrous capsule that at least partially separates them from adjacent organs and tissues. Two of these, the red bone marrow and thymus, are called primary lymphatic organs because lymphocytes mature here before colonizing the other sites. The other sites, called secondary lymphatic organs, are the lymph nodes, tonsils, and spleen.
Red bone marrow is a hemopoietic organ and the point of origin of all immune cells of the lymphatic system. It consists of islands of hemopoietic tissue composed of macrophages and developing blood cells, separated by sinusoids that converge on a central longitudinal vein. Lymphocytes and other formed elements pass from the islands into the sinusoids and enter the bloodstream.
The thymus is located in the mediastinum above the heart. It is divided into numerous polygonal lobules, each with a dense cortex and lighter medulla. Reticular epithelial cells separate the cortex from the medulla and surround the blood vessels, forming a blood–thymus barrier that isolates developing lymphocytes from blood-borne antigens. These cells also secrete thymic hormones that regulate T cell development and activity.
Lymph nodes are numerous small, bean-shaped organs that receive lymph through afferent lymphatic vessels, filter it, and pass it along via efferent lymphatic vessels that exit the hilum. They are the only lymphatic organs with both afferent and efferent lymphatic vessels. They monitor the lymph for foreign antigens, remove impurities before it returns to the bloodstream, contribute lymphocytes to the lymph and blood, and mount immune responses to foreign antigens.
The parenchyma of a lymph node exhibits an outer cortex composed mainly of lymphatic nodules, and a deeper medulla with a network of medullary cords. B cells multiply and differentiate into plasma cells in the germinal centers of the nodules. T cells are very concentrated in the deep cortex next to the medulla.
Lymph nodes are widespread but especially concentrated in cervical, axillary, thoracic, abdominal, intestinal, mesenteric, inguinal, and popliteal groups.
The tonsils encircle the pharynx and guard against inhaled and ingested pathogens. They include a median pharyngeal tonsil in the nasopharynx, a pair of palatine tonsils at the rear of the oral cavity, and numerous lingual tonsils clustered in the root of the tongue. Their superficial surface is covered with epithelium and their deep surface with a fibrous partial capsule. They have deep pits called tonsillar crypts bordered by rows of lymphatic nodules.
The spleen lies in the left hypochondriac region between the diaphragm, stomach, and kidney. Its parenchyma is composed of red pulp containing concentrated RBCs and white pulp composed of lymphocytes and macrophages. The spleen monitors the blood for foreign antigens, activates immune responses to them, disposes of old RBCs, and helps to regulate blood volume.
22.3 The Lymphatic System in Relation to Immunity (p. 623) The immune system is a population of disease-fighting cells (mainly lymphocytes and macrophages) that inhabit the mucous membranes, connective tissues, and lymphatic organs among other sites.
The three lines of defense against pathogens are: (1) surface barriers to invasion (skin and mucous membranes); (2) various additional forms of nonspecific defense against pathogens that breach the surface; and (3) specific immunity.
Nonspecific defense includes the surface barriers, neutrophils and macrophages, NK cells, immune surveillance, inflammation, fever, and antimicrobial proteins. It does not provide a mechanism for remembering a particular pathogen.
Specific immunity is a rapid response to a particular pathogen remembered from a previous encounter. The two forms of specific immunity are humoral immunity, carried out by B cells and antibodies, and cellular immunity, carried out by cytotoxic T cells.
Both B and T cells arise from bone marrow pluripotent stem cells through lymphocyte colony-forming units. Before participating in immune reactions, they must become immunocompetentand survive the process of negative selection, which eliminates lymphocytes that would attack the body’s own tissues.
B cells achieve immunocompetence in the red bone marrow. The few that survive negative selection emigrate to colonize the lymph nodes and other sites, where they multiply and stand guard against pathogens.
Upon encountering an antigen, a B cell displays fragments of it on its surface. TH cells respond by secreting helper factors, which stimulate B cell multiplication. Most of the daughter B cells become antibody-secreting plasma cells, while some become the memory B cells that confer lasting immunity to that antigen.
T cells originate in the red bone marrow but achieve immunocompetence and undergo selection in the thymic cortex. Mature T cells then pass into the medulla, emigrate from the thymus via blood and lymphatic vessels, and colonize the various lymphatic organs. TC cells directly attack enemy cells and destroy them with a lethal hit of toxic chemicals. Some T cells remain as memory T cells that produce lasting immunity.
22.4 Developmental and Clinical Perspectives (p. 626) Lymphatic ducts begin as lymph sacs, which are endothelium-lined channels in the mesoderm. Some of these arise by budding from the blood vessels and then detaching from them, and some as isolated mesodermal channels that fuse with each other and then (in the case of the two collecting ducts) join the venous system.
Lymph nodes develop from embryonic lymph sacs that become infiltrated with lymphocytes, which form dense clusters and become encapsulated.
The spleen develops from a mass of mesenchymal cells in the dorsal mesentery leading to the stomach. Most of its development is postnatal.
In old age, the lymphatic system itself shows little anatomical change, but the quantity of lymphatic tissue and numbers of lymphocytes decline, and immune responses become less efficient. Elderly people are thus more susceptible to infectious diseases and cancer. Autoantibody levels rise in old age and cause such autoimmune diseases as rheumatoid arthritis.
There are three principal classes of immune system disorders: autoimmune diseases (misdirected immune attacks), hypersensitivity disorders such as allergy, and immunodeficiency diseases such as SCID and AIDS. AIDS, the most notorious immune disease, is caused by destruction of TH cells by HIV, leaving a person susceptible to cancer and a variety of opportunistic infections.