Lymphatic System and Immunity
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Chapter 16Lymphatic System and Immunity
The lymphatic and cardiovascular systems include a network of capillaries and vessels that assist in circulating the body fluids. The lymphatic vessels transport excess fluid away from the interstitial spaces of tissues and return it to the bloodstream. The walls of both vessels are alike. For instance, they both contain a single layer of epithelial cells that allows fluids and substances to cross into them.
The lymphatic capillary system is found next to the systemic and pulmonary capillary networks. It then travels through lymph vessels into lymph nodes. It returns to lymph vessels and then is returned into the bloodstream at various points.
The lymphatic trunks are named for the regions they serve. The locations can be found in fig. 16.4, on page 623. The collecting ducts are: Thoracic duct—It begins in the abdomen. It passes upward medially through the diaphragm to the left subclavian, where it empties. Right lymphatic duct—It begins as the union of the right jugular, right subclavian, and right bronchomediastinal trunks. It empties into the right subclavian vein.
Lymph is tissue fluid that has entered into a lymphatic capillary.
The primary functions of lymph are to return the proteins to the bloodstream that have leaked out of the blood capillaries and to transport bacteria and other foreign particles to the lymph nodes.
The contractions of the skeletal muscles, pressure changes due to the actions of breathing muscles, and smooth muscle contractions of the larger lymphatic trunks all aid in the movement of lymph through the body.
Continuous movement of fluid from the interstitial spaces into the lymphatic system stabilizes the volume of fluids in these spaces. When an obstruction occurs, the tissue fluid builds up and causes edema.
Each lymph node is enclosed in a capsule of fibrous connective tissue and subdivides into compartments. The compartments contain dense masses of lymphocytes and macrophages. These masses, called nodules, are the structural units of a lymph node. Lymph nodes function in lymphocyte production and phagocytosis of foreign substances, damaged cells, and cellular debris.
The major body regions include: cervical region, axillary region, inguinal region, pelvic cavity, abdominal cavity, thoracic cavity, and supratrochlear region.
The thymus is a soft, bilobed structure whose lobes are surrounded by a capsule of connective tissue. It is composed of lymphatic tissue, which is subdivided into lobules by connective tissues. The lobules contain many lymphocytes. It functions to produce T-lymphocytes that help in the immune response. It also secretes thymosin, which is thought to stimulate the maturation of T-lymphocytes after they leave the thymus.
The spleen is the largest lymphatic organ. It resembles a large lymph node and is subdivided into chambers or lobules. The spaces within the chambers are filled with blood instead of lymph. There are two types of tissues within the lobules of the spleen. They include: White pulp - distributed throughout the spleen in tiny islands, composed of splenic nodules, and containing large numbers of lymphocytes. Red pulp - surrounds the venous sinuses and contains many red blood cells along with numerous lymphocytes and macrophages. The spleen functions to filter the blood.
Nonspecific body defenses include species resistance, mechanical barriers such as the skin and mucous membranes, and chemical barriers such as enzymes, interferon, inflammation, and phagocytosis. Specific body defenses include immune mechanisms, where certain cells recognize the presence of particular foreign substances and act against them. Lymphocytes and macrophages achieve this.
Species resistance is referring to the fact that a given kind of organism or species develops diseases that are unique to it. A species may be resistant to diseases that affect other species, because its tissues somehow fail to provide the temperature or chemical environment needed by a particular pathogen.
The skin and the mucous membranes are two mechanical barriers to infection.
Enzymes provide a chemical barrier to pathogens. By splitting components of the pathogen or decreasing the pH, the enzyme can have lethal effects on pathogens.
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 provide broad protection against a wide variety of microbes by grabbing onto them.
Viral or bacterial infection stimulates certain lymphocytes to secrete IL-1, which temporarily raises body temperature. Physical factors, such as heat or ultraviolet light, or chemical factors, such as acids or bases, can cause fever. Elevated body temperature and the resulting decrease in blood iron level and increased phagocytic activity hamper infection.
NK cells are a small population of lymphocytes. NK cells defend the body against various viruses and cancer by secreting cytolytic substances called perforins.
Localized redness result of blood vessel dilation and the increase in blood volume of affected tissues. Swelling result of increased blood volume and increased permeability of nearby capillaries. Heat due to the presence of blood from deeper body parts, which is generally warmer than that near the surface. Pain results from the stimulation of nearby pain receptors.
The most active phagocytic cells of the blood are neutrophils and monocytes. Macrophages are fixed phagocytic cells found in lymph nodes, spleen, liver, and lungs. This constitutes reticuloendothelial tissue.
An antigen is a foreign substance, such as a protein, polysaccharide or a glycolipid, to which lymphocytes respond. A hapten is a molecule that by itself cannot stimulate the immune response. It must combine with a larger molecule.
T cells originate in the thymus. B cells are those processed in another part of the body, probably the fetal liver.
The lysosomal digestive process of phagocytosis of an invading bacterium releases antigens. They are moved to the macrophage's surface membrane. They are then displayed on the membrane with major histocompatibility complex. If the antigen then fits the helper T cell, it becomes activated. At this point, the helper T cell seeks out the appropriate T cell and by attaching to it, activates the T cell into a response. Cell-mediated immunity (CMI) is when a T cell, for example, attaches itself to antigen-bearing cells and interacts with the foreign cells directly.
Cytokines (lymphokines) are a variety of polypeptides that are synthesized and secreted by T cells and macrophages. These enhance various cellular responses to antigens. They stimulate the synthesis of lymphokines from other T cells, help activate resting T cells, cause T cells to proliferate, stimulate the production of leukocytes in the red bone marrow, cause growth and maturation of B cells, and activate macrophages.
Clone of lymphocytes refers to cells that are derived from one early cell that are capable of responding to a certain antigen. As there are many differing antigens, there are also many differing varieties of clones.
A B cell is activated when it binds to an activated T cell. An activated B cell proliferates, enlarging its clone. Some activated B cells specialize into antibody-producing plasma cells. Antibodies react against the antigen-bearing agent that stimulated their production. An individual’s diverse B cells defend against a very large number of pathogens.
B cells become activated when they encounter an antigen whose molecular shape fits the shape of the B cell's antigen receptors. As a result of this combination, the B-cells proliferate by mitosis and its clone is enlarged. This mechanism for activation is similar to the lock and key model used by enzymes and substrates.
Plasma cells are some of the newly formed members of the activated B cell's clone. They make use of their DNA information and protein-synthesizing mechanism to produce antibody molecules.
An immunoglobulin molecule consists of two identical light changes of amino acids and two identical heavy chains of amino acids. See figure 16.20, page 637.
Variable regions are the portion of one end of each of the heavy and light chains consists of variable sequences of amino acids making them specific for specific antigen molecules. Constant regions are the remaining portions of the chains whose amino acid sequences are very similar from molecule to molecule.
Immunoglobulin G (IgG)—occurs in plasma and tissue fluids. Immunoglobulin A (IgA)—occurs in milk, tears, nasal fluid, gastric juice, intestinal juice, bile, and urine. Immunoglobulin M (IgM)—develops in blood plasma. Immunoglobulin D (IgD)—is important in activating B cells. Immunoglobulin E (IgE)—occurs in exocrine secretions and is associated with allergic reactions.
Agglutination—antibodies combine with antigens and clumping results. Precipitation—antibodies combine with antigens and insoluble substance forms. Neutralization—antibodies cover the toxic portions of antigen molecules and neutralize their effects. Lysis—antibodies cause the cell membranes to rupture.
It is a group of inactive enzymes that become activated when certain IgG or IgM antibodies combine with antigens and the reactive sites become exposed. The activated enzymes produce chemotaxis, agglutination, opsonization, and lysis. It can also promote the inflammation reaction.
A primary immune response occurs when B cells or T cells become activated after first encountering the antigens to which they are specifically reactant. A secondary immune response happens when memory cells are activated and increased in size, so they can respond rapidly to the antigen to which they were previously sensitized.
Active immunity can be either naturally acquired or artificially acquired. Naturally acquired active immunity is stimulated as a result of exposure to live pathogens. Artificially acquired active immunity is stimulated by exposure to a vaccine containing weakened or dead pathogens. Passive immunity can also be either naturally acquired or artificially acquired. Naturally the antibodies passed to a fetus from a mother with active immunity stimulate acquired passive immunity. Artificially acquired passive immunity is stimulated by an injection of gamma globulin that contains antibodies.
A vaccine is a substance that contains an antigen that can stimulate a primary immune response against a particular disease-causing agent, but does not cause severe disease symptoms.
A vaccine contains bacteria or viruses that have been killed or weakened so they cannot cause a serious infection; or it may contain a toxin of an infectious organism that has been chemically altered to destroy its toxic effects. The antigens present still retain the characteristics needed to simulate a primary immune response.
Receptor-mediated endocytosis utilizing receptor sites on cells of the fetal yolk sac transfers IgG molecules to the fetus.
Allergic reactions are closely related to immune responses in that both may involve the sensitizing of lymphocytes or the combining of antigens with antibodies. Allergic reactions are likely to be excessive and to cause tissue damage.
An antigen is a substance that stimulates cells to produce antibodies. An allergen is a foreign substance capable of stimulating an allergic reaction.
In an immediate-reaction allergy, the individuals have an inherited ability to synthesize abnormally large quantities of antibodies in response to certain antigens. In this instance, the allergic reaction involves the activation of B-cells.
It results from repeated exposure of the skin to certain chemical substances. As a consequence of these repeated contacts, the foreign substance and a large number of T cells collect in the skin and eventually activate the T cells. Their actions and the actions of macrophages they attract cause the release of various chemical factors. This causes eruptions and inflammation of the skin. It is called delayed since it takes about forty-eight hours to occur.
Tissue rejection is when the immune system sees transplanted tissue as foreign and starts the immune response to try to rid the body of it.
Matching the donor and recipient tissues may reduce it. It can also involve giving drugs that suppress the immune system.
An immunosuppressive drug interferes with the recipient’s immune response by suppressing formation of antibodies or production of T cells. This will ultimately leave the recipient relatively unprotected against infection.
Autoimmunity occurs when the immune system does not distinguish between self and nonself and manufactures autoantibodies that attack the body's own cells. For whatever reason, the autoantibodies treat a certain cell type in the body as a foreign object and signal the immune system to defend against the perceived invader.
The immune system begins to decline early in life, in part due to the decreasing size of the thymus. Numbers of T cells and B cells do not change significantly, but activity levels do. Proportions of the different antibody classes shift. Yüklə 28,2 Kb. Dostları ilə paylaş:
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