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Chapter Summary 16.1 Overview of the Lymphatic System The lymphatic system returns excess extracellular fluid to the cardiovascular system, provides immunity, and aids in the digestion of fats. Masses of lymphocytes are found in lymph nodes and help fight infectious agents. 16.2 Lymphatic Pathways Lymphatic capillaries Lymphatic capillaries are microscopic, closed-ended tubes that extend into interstitial spaces. They receive tissue fluid through their thin walls. Lymphatic vessels Lymphatic vessels are formed by the merging of lymphatic capillaries. They have walls similar to veins, only thinner, and possess valves that prevent backflow of lymph. The larger lymphatic vessels lead to lymph nodes and then merge into lymphatic trunks. Lymphatic trunks and collecting ducts Lymphatic trunks drain lymph from large body regions. Trunks lead to two collecting ducts—the thoracic duct and the right lymphatic duct. Collecting ducts empty into the subclavian veins.
16.3 Tissue Fluid and Lymph Tissue fluid formation Tissue fluid originates from plasma and includes water and dissolved substances that have passed through the blood capillary wall. Tissue fluid generally lacks large proteins, but some smaller proteins are filtered out of blood capillaries into interstitial spaces. As the protein concentration of tissue fluid increases, colloid osmotic pressure increases. Lymph formation Increasing hydrostatic pressure in interstitial spaces forces some tissue fluid into lymphatic capillaries. This fluid becomes lymph. Lymph formation prevents accumulation of excess tissue fluid (edema). Lymph flow Lymph is under relatively low hydrostatic pressure and may not flow readily without external aid. Contraction of skeletal muscles, contraction of smooth muscle in the walls of the large lymphatic trunks, and low pressure in the thorax created by breathing movements move lymph. Any condition that interferes with the flow of lymph results in edema. Lymph function
The thymus is a soft, bilobed organ within the mediastinum. It slowly shrinks after puberty. It is composed of lymphatic tissue subdivided into lobules. Lobules contain lymphocytes. T lymphocytes leave the thymus and provide immunity. The thymus secretes thymosins, which stimulate maturation of T lymphocytes. Spleen The spleen is in the upper left portion of the abdominal cavity. It resembles a large lymph node encapsulated and subdivided into lobules by connective tissue. Spaces in splenic lobules are filled with blood. The spleen, which filters foreign particles and damaged red blood cells from the blood, contains many macrophages and lymphocytes. 16.5 Immunity: Body Defenses Against Infection The presence and reproduction of pathogens may cause an infection. Pathogens include bacteria, protozoa, fungi, and viruses. The body has an immunity to infection that involves both innate (nonspecific) and adaptive (specific) defenses. Innate (nonspecific) defenses
Species resistance Each species is resistant to certain diseases that may affect other species but is susceptible to diseases other species may resist. Mechanical barriers Mechanical barriers include the skin and mucous membranes. Intact mechanical barriers prevent entrance of some pathogens. Hair traps infectious agents. Fluids such as tears, sweat, saliva, mucus, and urine wash away microorganisms before they can firmly attach. Inflammation Inflammation is a tissue response to damage, injury, or infection. The response includes localized redness, swelling, heat, and pain. Chemicals released by damaged tissues attract white blood cells to the site. Clotting may occur in body fluids that accumulate in affected tissues. Connective tissue may form a sac around the injured tissue and thus aid in preventing the spread of pathogens. Chemical barriers Enzymes in gastric juice and tears kill some pathogens. Low pH in the stomach prevents growth of some bacteria. High salt concentration in perspiration kills some bacteria. Interferons stimulate uninfected cells to synthesize antiviral proteins that block proliferation of viruses, stimulate phagocytosis, and enhance activity of cells that help resist infections and stifle tumor growth. Defensins make holes in bacterial cell walls and membranes. Collectins broadly protect against a wide variety of microbes by binding to them, facilitating phagocytosis. Activation of complement proteins in plasma stimulates inflammation, attracts phagocytes, and enhances phagocytosis.
Varieties of T cells and B cells number in the millions. The members of each variety respond only to a specific antigen. Originating from a single cell, the members of each variety form a clone. 664 T cells and the cellular immune response T cells are activated when an antigen-presenting cell displays a foreign antigen. When a macrophage acts as an accessory cell, it phagocytizes an antigen-bearing agent, digests the agent, and displays the resulting antigens on its cell membrane in association with certain MHC proteins. T cells respond to antigens by cell-to-cell contact (cellular immune response). T cells secrete cytokines, such as interleukins, that enhance cellular responses to antigens and stimulate proliferation of a T cell to enlarge its clone. T cells may also secrete substances that are toxic to their target cells. A helper T cell becomes activated when it encounters displayed antigens for which it is specialized to react. Once activated, a helper T cell stimulates a type of B cell (plasma cell) to produce antibodies for the specific antigen. Cytotoxic T cells recognize foreign antigens on tumor cells and cells whose surfaces indicate that they are infected by viruses. Stimulated cytotoxic T cells secrete perforin to destroy these cells. Memory T cells allow for immediate response to second and subsequent exposure to the same antigen. B cells and the humoral immune response Sometimes a B cell is activated when it encounters an antigen that fits its antigen receptors, or more often a B cell is activated when stimulated by a helper T cell. An activated B cell proliferates (especially when stimulated by a T cell), enlarging its clone. Some activated B cells differentiate into antibody-producing plasma cells. Antibodies react against the antigen-bearing agent that stimulated their production (humoral immune response). An individual’s many different B cells defend against many pathogens.
Other activated B cells differentiate further into memory B cells. Antibodies are soluble proteins called immunoglobulins. They constitute the gamma globulin fraction of plasma. Each immunoglobulin molecule consists of four linked chains of amino acids. Variable regions at the ends of these chains are specialized antigen-binding sites that react with different antigens. The five major types of immunoglobulins are IgG, IgA, IgM, IgD, and IgE. IgG, IgA, and IgM make up most of the circulating antibodies. Antibodies directly attack antigens, or bind antigens to activate complement, or stimulate local tissue changes that are unfavorable to antigen-bearing agents. Direct attack results in agglutination, precipitation, or neutralization. Activated complement proteins alter infected cells so they become more susceptible to phagocytosis, attract phagocytes, and lyse foreign cell membranes. Immune responses B cells and T cells first encountering an antigen for which they are specialized to react constitutes a primary immune response. (a) During this response, antibodies are produced for several weeks. (b) Some T cells and B cells remain dormant as memory cells. A secondary immune response occurs as memory cells rapidly respond to subsequent exposure to an antigen. Practical classification of immunity A person who encounters a pathogen and has a primary immune response develops naturally acquired active immunity. A person who receives a vaccine containing a dead or weakened pathogen, or part of it, develops artificially acquired active immunity. Herd immunity protects populations. A person who receives an injection of antiserum or antitoxin has artificially acquired passive immunity.
Matching cell surface molecules (MHC antigens) of donor and recipient tissues, transplanting stem cells from the donor, and the use of immunosuppressive drugs can help the body accept the foreign tissue. Autoimmunity In autoimmune disorders, autoantibodies attack the body’s tissues. Autoimmune disorders may result from a previous viral infection, faulty T cell development, or reaction to a nonself antigen that resembles a self antigen. Retained fetal cells can cause a condition that resembles an autoimmune disorder. 16.6 Life-Span Changes The immune system begins to decline early in life, in part due to the shrinking thymus. Numbers of T cells and B cells do not significantly change, but activity levels do. Proportions of the different antibody classes shift.
