Ch. 16 Lymphatic System

Thymus Histology

- Connective tissues extend inward from the surface of the thymus, subdividing it into lobules. The lobules house many lymphocytes that developed from progenitor cells in the bone marrow. Most of these cells (thymocytes) are inactive; however, some mature into T lymphocytes, also called T cells, which leave the thymus and provide immunity. Epithelial cells in the thymus secrete protein hormones called thymosins, which stimulate maturation of T lymphocytes.

gen

become, be produced: allergen—substance that evokes an allergic response.

Natural Killer Cells (NK)

cells are a small population of lymphocytes that are distinctly different from the lymphocytes that provide adaptive defense mechanisms. NK cells defend the body against various viruses and cancer cells by secreting cytolytic ("cell-cutting") substances called perforins that lyse the cell membrane, destroying the infected cell. NK cells also secrete chemicals that enhance inflammation.

patho

disease, sickness: pathogen—disease-causing agent.

immun

free, exempt: immunity—resistance to (freedom from) a specific disease.

humor

moisture, fluid: humoral immunity—immunity resulting from antibodies in body fluids

auto

self: autoimmune disease—the immune system attacking the body's own tissues.

nod

knot: lymph nodule—small mass of lymphocytes surrounded by connective tissue.

T-Cells and Cellular Immune Response

- A lymphocyte must be activated before it can respond to an antigen. T cell activation requires that processed fragments of the antigen be attached to the surface of another type of cell, called an antigen-presenting cell (accessory cell). Dendritic cells located in the epidermis, mucous membrane, and lymphatic organs may engulf the antigen by receptor-mediated endocytosis, process the antigen, and display antigen fragments on their cell surfaces. Macrophages and B cells can also be antigen-presenting cells. T cell activation may occur when a macrophage phagocytizes a bacterium, and digests it within a phagolysosome formed by the fusion of the vesicle containing the bacterium (phagosome) and a lysosome. Some of the resulting bacterial antigens are displayed on the macrophage's cell membrane near certain protein molecules that are part of a group of proteins called the major histocompatibility complex (MHC) or human leukocyte antigens (HLA) because they were first identified on white blood cells. MHC antigens help T cells recognize that a newly displayed antigen is foreign, not self. Class I MHC antigens are in cell membranes of all body cells except red blood cells. Class II MHC antigens are on the surfaces of antigen-presenting cells, thymus cells, and activated T cells. Activated T cells interact directly with the antigen-presenting cell. Such cell-to-cell contact is called the cellular immune response, or cell-mediated immunity.

Helper T Cells

- A particularly important specialized type of T cell, called a helper T cell, is activated when its antigen receptor combines with a displayed foreign antigen. Once activated, the helper T cell proliferates and the resulting cells stimulate a type of B cell (plasma cell) to produce antibodies that are specific for the displayed antigen. A subtype of helper T cell called a CD4 cell is the prime target of HIV, the virus that causes AIDS. (CD4 stands for the "cluster-of-differentiation" antigen the T cell bears that enables it to recognize a macrophage displaying a foreign antigen.) Considering the central role of CD4 helper T cells in establishing immunity—they stimulate B cells and secrete cytokines—it is no wonder that when HIV harms them, immunity declines. Another type of T cell is a cytotoxic T cell, which recognizes and combines with nonself antigens that cancerous cells or virally infected cells display on their surfaces near certain MHC proteins. Cytokines from helper T cells activate the cytotoxic T cell Next the cytotoxic T cell proliferates, enlarging its clone of cells. Cytotoxic T cells then bind to the surfaces of antigen-bearing cells, where they release perforin protein that cuts porelike openings, destroying these cells. In this way, cytotoxic T cells continually monitor the body's cells, recognizing and eliminating tumor cells and cells infected with viruses.

Antibody Molecules

- Antibodies are soluble, globular proteins that constitute the gamma globulin fraction of plasma proteins. Each antibody molecule consists of four chains of amino acids linked by pairs of sulfur atoms that attract by disulfide bonds. The four chains form a Y-shaped structure. Two of these amino acid chains are identical light chains (L-chains), and two are identical heavy chains (H-chains). The heavy chains have about twice as many amino acids as the light chains. The five major types of antibody molecules are distinguished by a particular type of heavy chain. Most of the types of antibody molecules consist of a single Y-shaped structure, but some have as many as five

Antigens

- Before birth, cells inventory the proteins and other large molecules in the body, learning to identify these as self. The lymphatic system responds to nonself antigens, but not normally to self antigens. Receptors on lymphocyte surfaces enable these cells to recognize nonself antigens. Antigens may be proteins, polysaccharides, glycoproteins, or glycolipids. The antigens most effective in eliciting an immune response are large and complex, with few repeating parts. Sometimes, a smaller molecule that cannot by itself stimulate an immune response combines with a larger one, which makes it able to do so (become antigenic). Such a small molecule is called a hapten. Stimulated lymphocytes react either to the hapten or to the larger molecule of the combination. Hapten molecules are in drugs, such as penicillin; in household and industrial chemicals; in dust particles; and in animal dander.

Lymph Node Histology

- Blood vessels and nerves join a lymph node through the indented region of the lymph node, called the hilum. The lymphatic vessels leading to a lymph node (afferent vessels) enter separately at various points on its convex surface, but the lymphatic vessels leaving the lymph node (efferent vessels) exit from the hilum. The capsule surrounding the lymph node extends into the lymph node and partially subdivides it into compartments called lymphatic nodules, the functional units of the lymph node. Masses of B lymphocytes (B cells) and macrophages in the cortex are contained within these nodules, also called lymphatic follicles. Within the lymph nodules are germinal centers where B lymphocytes proliferate. The medulla contains mostly T lymphocytes (T cells). Spaces in a lymph node, called lymphatic sinuses, provide a complex network of chambers and channels through which lymph circulates. Lymph enters a lymph node through afferent lymphatic vessels, moves slowly through the lymph sinuses, and leaves through efferent lymphatic vessels

Tissue Fluid Formation

- Capillary blood pressure filters water and small molecules from the plasma. The resulting fluid has much the same composition as the plasma (including nutrients, gases, and hormones), with the important exception of the plasma proteins, most of which are too large to pass through the blood capillary walls. The osmotic effect of these plasma proteins (called the plasma colloid osmotic pressure) helps to draw fluid back into the blood capillaries by osmosis.

Memory T-Cells

- Certain cytotoxic T cells, called CD8 T cells, give rise to memory T cells that provide for future immune protection. When a CD8 T cell contacts an antigen-presenting cell, it contorts into a dumbbell shape. The side of the dumbbell that contacts the antigen-presenting cell accumulates different receptors and other proteins from the side facing farthest from the provoking antigen. When the CD8 T cell divides, the daughter cell that was the part of the original cell closest to the antigen becomes an active cytotoxic T cell. The daughter cell farther from the antigen becomes a memory T cell. As its name implies, a memory T cell does not respond to an initial exposure to an antigen, but upon subsequent exposure immediately divides and differentiates into a cytotoxic T cell. This memory cell response usually destroys the pathogen before the body responds to it with the signs and symptoms of disease.

Spleen Histology

- During fetal development, pulp cells of the spleen produce blood cells, much as red bone marrow cells do after birth. As the time of birth approaches, the spleen stops producing red blood cells. However, in certain diseases, such as erythroblastosis fetalis, in which many red blood cells are destroyed, the splenic pulp cells may resume their hematopoietic activity. The venous sinuses in the red pulp are quite permeable. Red blood cells can squeeze through pores in the sinus walls and enter the surrounding spaces. The older, more fragile red blood cells may rupture during this passage. Macrophages in the venous sinuses remove the resulting cellular debris. Macrophages engulf and destroy foreign particles, such as bacteria, that may be carried in the blood as it flows through the venous sinuses. Lymphocytes of the spleen, like those of the thymus and lymph nodes, also help defend the body against infections. Thus, the spleen filters blood much as the lymph nodes filter lymph.

Clone

- Each person has millions of varieties of T and B cells. The members of each variety originate from a single early cell, so they are all alike, forming a clone of cells (genetically identical cells originating from division of a single cell). The members of each variety have a particular type of antigen receptor on their cell membranes that can respond only to a specific antigen.

Chemical Barriers

- Enzymes in body fluids provide a chemical barrier to pathogens. Gastric juice, for example, contains the protein-splitting enzyme pepsin and has a low pH due to hydrochloric acid in the stomach. The combined effect of pepsin and hydrochloric acid kills many pathogens that enter the stomach. Similarly, tears contain the enzyme lysozyme, which destroys certain bacteria on the eyes. The accumulation of salt from perspiration kills certain bacteria on the skin. Interferons are proteins that lymphocytes and fibroblasts produce in response to viruses or tumor cells. Once released from a virus-infected cell, interferon binds to receptors on uninfected cells, stimulating them to synthesize proteins that block replication of a variety of viruses. Thus, interferon's effect is nonspecific. Interferons also stimulate phagocytosis and enhance the activity of other cells that help to resist infections and the growth of tumors.

Lymph Formation

- Filtration from the plasma normally exceeds reabsorption, leading to the net formation of tissue fluid. This accumulation of tissue fluid increases the tissue fluid hydrostatic pressure, the force which moves tissue fluid into lymphatic capillaries, forming lymph. In this way, lymph formation prevents the accumulation of excess tissue fluid, or edema Lymph contains: Nutrients, Hormones, Gasses

Immunoglobulin A (IgA)

- Immunoglobulin A (IgA) is in exocrine gland secretions. It is in tears, nasal fluid, gastric juice, intestinal juice, bile, and urine. A newborn receives IgA from colostrum, a yellowish fluid that the mother's breasts secrete for the first few days after giving birth, and then from breast milk. Antibodies in colostrum and breast milk protect against certain digestive and respiratory infections.

Immunoglobulin D (IgD)

- Immunoglobulin D (IgD) is on the surfaces of most B cells, especially those of infants. IgD acts as an antigen receptor and is important in activating B cells

Immunoglobulin G (IgG)

- Immunoglobulin G (IgG) is in plasma and tissue fluids and is effective against bacteria, viruses, and toxins. IgG also activates complement proteins. Anti-Rh antibodies are examples of IgG and can cross the placenta, as described in section 14.5, Blood Groups and Transfusions, Rh Blood Group. A newborn does not yet have its own antibodies but has IgG that passed through the placenta from the mother that protect the infant against some illnesses to which the mother is immune. These maternally obtained antibodies begin to disappear just about when the infant begins to make its own antibodies.

Immunoglobulin M (IgM)

- Immunoglobulin M (IgM) is a type of antibody produced in plasma in response to contact with certain antigens in foods or bacteria. Examples of IgM are the anti-A and anti-B antibodies, described in

Location of Lymph Nodes

- Lymph nodes are found in groups or chains along the paths of the larger lymphatic vessels throughout the body, but they are not in the central nervous system. 1. The lymph nodes in the cervical region follow the lower border of the mandible, anterior to and posterior to the ears, and lie deep in the neck along the paths of the larger blood vessels. These nodes are associated with the lymphatic vessels that drain the skin of the scalp and face, as well as the tissues of the nasal cavity and pharynx. The illness described as "swollen glands" refers to enlarged cervical lymph nodes associated with throat or respiratory infection. 2. Lymph nodes in the axillary region receive lymph from vessels that drain the upper limbs, the wall of the thorax, the mammary glands (breasts), and the upper wall of the abdomen. 3. The lymph nodes in the supratrochlear region are located superficially on the medial side of the elbow. They often enlarge in children in response to infections acquired through cuts and scrapes on the hands. 4. Lymph nodes in the inguinal region receive lymph from the lower limbs, the external genitalia, and the lower abdominal wall. 5. In the pelvic cavity lymph nodes primarily follow the iliac blood vessels. They receive lymph from the lymphatic vessels of the pelvic viscera. 6. The lymph nodes in the abdominal cavity form chains along the main branches of the mesenteric arteries and the abdominal aorta and receive lymph from the abdominal viscera. 7. The lymph nodes in the thoracic cavity are in the mediastinum and along the trachea and bronchi. They receive lymph from the thoracic viscera and from the internal wall of the thorax.

Lymph Node Function

- Lymph nodes have two primary functions: filtering potentially harmful particles from lymph before returning it to the bloodstream and monitoring body fluids (immune surveillance) through the actions of lymphocytes and macrophages. Lymphocytes in the lymph nodes attack viruses, bacteria, and parasitic cells that are brought to the lymph nodes by lymph in lymphatic vessels. Macrophages in the lymph nodes engulf and destroy foreign substances, damaged cells, and cellular debris.

Complement

- is a group of proteins (complement system), in plasma and other body fluids, that interact in an expanding series of reactions or cascade. Complement activation can rapidly occur by the classical pathway when a complement protein binds to an antibody attached to its specific antigen (discussed later in this chapter, in the section "Antibody Actions"), or more slowly by the alternative pathway triggered by exposure to foreign antigens, in the absence of antibodies. Activation of complement stimulates inflammation, attracts phagocytes, and enhances phagocytosis.

Lymph Flow

- Lymph within lymphatic vessels, like venous blood, is under relatively low hydrostatic pressure. It may not flow readily through the lymphatic vessels without help from contracting skeletal muscles in the limbs, contraction of smooth muscle in the walls of the larger lymphatic trunks, and pressure changes from the action of skeletal muscles used in breathing. Lymph flow peaks during physical exercise, due to the actions of skeletal muscles and pressure changes associated with breathing. Contracting skeletal muscles compress lymphatic vessels. This squeezing action moves the lymph inside lymphatic vessels. Valves in these vessels prevent backflow, so lymph can only move toward a collecting duct. Additionally, the smooth muscle in the walls of the larger lymphatic trunks contracts rhythmically and compresses the lymph inside, forcing the fluid onward. Breathing aids lymph circulation by creating a relatively low pressure in the thorax during inhalation. At the same time, the contracting diaphragm increases the pressure in the abdominal cavity. Consequently, lymph is squeezed out of the abdominal vessels and forced into the thoracic vessels. Once again, the valves of the lymphatic vessels prevent lymph backflow. Conditions that interfere with lymph movement cause tissue fluid to accumulate in interstitial spaces, producing edema. This can happen when the extent of a breast tumor requires that the surgeon remove nearby axillary lymph nodes, which can obstruct drainage from the upper limb, causing edema

Lymph Function

- Lymphatic capillaries in the small intestine play a major role in the absorption of dietary fats Lymph also returns to the bloodstream most of the small proteins that the blood capillaries filtered. At the same time, lymph transports foreign particles, such as bacteria and viruses, to lymph nodes. Lymphatic capillaries are adapted to receive proteins and foreign particles in a way that blood capillaries are not. The epithelial cells that form the walls of lymphatic capillaries overlap but are not attached to each other. The valves are pushed inward when the hydrostatic pressure is greater on the outside of the lymphatic capillary but close when the hydrostatic pressure is greater on the inside.

Lymphatic Tissues and Organs

- Lymphatic tissue contains lymphocytes, macrophages, T-Cells, B-Cells. The unencapsulated diffuse lymphatic tissue associated with the digestive, respiratory, urinary, and reproductive tracts is called the mucosa-associated lymphoid tissue (MALT). Included in the MALT are compact masses of lymphatic tissue, called lymphatic nodules, which comprise the tonsils and appendix. The lymphatic organs, including the lymph nodes, thymus, and spleen, are encapsulated lymphatic tissue. A capsule of connective tissue with many fibers encloses each organ.

Lymphocyte Origins

- Lymphocyte production begins during fetal development and continues throughout life, with red bone marrow releasing unspecialized precursors of lymphocytes into the circulation. About half of these cells reach the thymus, where they remain for a time. Here, these thymocytes specialize into T lymphocytes, or T cells. (T refers to thymus-derived lymphocytes.) Some of these T cells constitute 70% to 80% of the circulating lymphocytes in blood. Other T cells reside in lymphatic organs and are particularly abundant in the lymph nodes, the thoracic duct, and the white pulp of the spleen. Other lymphocytes remain in the red bone marrow until they differentiate fully into B lymphocytes, or B cells. (Historically, the "B" stands for bursa of Fabricius, an organ in the chicken where these cells were discovered.) The blood distributes B cells, which constitute 20% to 30% of circulating lymphocytes (fig. 16.16). B cells settle in lymphatic organs along with T cells and are abundant in the lymph nodes, spleen, bone marrow, and intestinal lining

Defensins and Collectins

- Other antimicrobial biochemicals are defensins and collectins. Defensins are peptides produced by neutrophils and other types of granular white blood cells in the intestinal epithelium, the urogenital tract, the kidneys, and the skin. Recognition of a nonself cell surface or viral particle triggers the expression of genes that encode defensins. Some defensins make holes in bacterial cell walls and membranes, crippling the microbes. Collectins are proteins that provide broad protection against bacteria, yeasts, and some viruses. These proteins home in on slight differences in the structures and arrangements of sugars that protrude from the surfaces of pathogens. Collectins detect not only the sugar molecules, but the pattern in which they are clustered, binding much like velcro clings to fabric, thus making the pathogen more easily phagocytized.

Immunity

- The ability of the human body to prevent entry of pathogens or destroy them if they enter is called immunity. Some immune mechanisms are general in that they protect against many types of pathogens, providing innate (nonspecific) defense. These general responses function the same way regardless of the type of pathogen or the number of exposures. Such mechanisms include species resistance, mechanical barriers, inflammation, chemical barriers (enzyme action, interferon, and complement), natural killer cells, phagocytosis, and fever. Other protective mechanisms are very precise, targeting specific pathogens with an adaptive (specific) defense. These more directed responses are carried out by specialized lymphocytes that recognize foreign molecules (nonself antigens) in the body and act against them. Together, innate and adaptive defense mechanisms protect the body against infection. The innate defenses respond quite rapidly, whereas adaptive defenses develop more slowly.

Pathogens

- The organs of the lymphatic system also help defend the body against infection by disease-causing agents, or pathogens Pathogens include simple microorganisms such as bacteria, complex microorganisms such as protozoa, and spores of multicellular organisms such as fungi. Viruses are pathogens, but they are not considered organisms, because their structure is much simpler than that of a living cell and they must infect a living cell to reproduce.

Antigen Binding Sites

- The sequences of amino acids of the heavy and light chains confer the unique, three-dimensional structure (conformation) of each type of antibody, as is the case for other proteins. This special conformation, in turn, imparts the physiological properties of the molecule. One end of each of the heavy and light chains consists of variable sequences of amino acids (variable regions). These regions are specialized to fit the shape of a specific antigen molecule. Antibodies can bind to certain antigens because of the conformation of the variable regions. The antibody contorts, forming a pocket around the antigen. These specialized ends of the antibody molecule are called antigen-binding sites, and the parts that bind the antigen are called idiotypes

Mechanical Barriers

- The skin and mucous membranes lining the passageways of the respiratory, digestive, urinary, and reproductive systems create mechanical barriers that prevent the entrance of some infectious agents and provide a first line of defense. As long as these barriers remain intact, many pathogens are unable to penetrate them. Another protection is that the epidermis sloughs off, removing superficial bacteria with it. Organs of the Respiratory System, Trachea, that lines the respiratory passages entraps particles and sweeps them out of the airways and into the pharynx, where they are swallowed. Hair traps infectious agents associated with the skin and mucous membranes, and sweat and mucus rinse away microorganisms. Tears, saliva, and urine also wash away microorganisms before they become firmly attached. The rest of the nonspecific defenses discussed in this section are part of the second line of defense.

Spleen

- The spleen is the largest lymphatic organ. It is in the upper left portion of the abdominal cavity, just inferior to the diaphragm, posterior and lateral to the stomach The tissues in the lobules of the spleen are of two types. White pulp is distributed throughout the spleen in tiny islands. This tissue is composed of splenic nodules, which are similar to the lymphatic nodules in lymph nodes and are packed with lymphocytes. The red pulp, which fills the remaining spaces of the lobules, includes the venous sinuses and the space around the venous sinuses. This pulp contains abundant red blood cells, which impart its color, plus many lymphocytes and macrophages

Adaptive Specific Defenses

- The third line of defense is resistance to specific pathogens or to their toxins or metabolic by-products. This response is based upon the ability to distinguish molecules that are part of the body ("self") from those that are not ("nonself," or foreign). Such nonself molecules that can elicit an immune response are called antigens (an′tĭ-jenz). Lymphocytes and macrophages that recognize specific nonself antigens carry out adaptive immune responses, which include the cellular immune response and humoral immune response

Thoracic Duct

- The thoracic duct is the wider and longer of the two collecting ducts. It originates as an enlarged sac, the cisterna chyli in the abdomen, and passes upward through the diaphragm beside the aorta. The thoracic duct ascends anterior to the vertebral column through the mediastinum, and empties into the left subclavian vein near the junction of the left jugular vein. This duct drains lymph from the intestinal, lumbar, and intercostal trunks, as well as from the left subclavian, left jugular, and left bronchomediastinal trunks. The thoracic duct empties into the left subclavian vein near its junction with the left jugular vein. Drains into the Thoracic Duct: - Left Lower Limb - Left Upper Limb - Abdomen - Left side of head/neck - Right Lower Limb

Thymus

- The thymus is a soft, bilobed gland enclosed in a connective tissue capsule. It is in the mediastinum, anterior to the aortic arch and posterior to the upper part of the body of the sternum, and extends from the root of the neck to the pericardium. The thymus varies in size and is usually proportionately larger during infancy and early childhood. After puberty, the thymus shrinks, and in an adult, it may be small

Lymphatic Vessels

- The walls of lymphatic vessels are similar to those of veins, but thinner. Each lymphatic vessel is composed of three layers: an endothelial lining, a middle layer of smooth muscle and elastic fibers, and an outer layer of connective tissue. The lymphatic vessels are also like some peripheral veins in that they have semilunar valves, which help prevent backflow of lymph --> The larger lymphatic vessels lead to specialized organs called lymph nodes. These vessels merge into larger lymphatic trunks after leaving the lymph nodes.

Types of Immunoglobulins

- Three of the five major types of immunoglobulins include most of the circulating antibodies. These types are immunoglobulin G, which accounts for about 80% of the antibodies; immunoglobulin A, which makes up about 13%; and immunoglobulin M, responsible for about 6%. The remainder of the antibodies are immunoglobulin D or immunoglobulin E.

B Cells and Humoral Immune Response

- When a B cell encounters an antigen whose molecular shape fits the shape of the B cell's antigen receptors, it becomes activated. In response to the receptor-antigen combination, the B cell divides repeatedly. Such B cell activation typically requires T cell "help." When an activated helper T cell encounters a B cell already combined with an identical foreign antigen, the helper T cell releases certain cytokines that stimulate the B cell to proliferate, enlarging its clone of cells. The cytokines also attract macrophages and leukocytes into inflamed tissues and help keep them there. Some members of the activated B cell's clone differentiate further into plasma cells, which produce and secrete large globular proteins called antibodies, also termed immunoglobulins. Antibodies are similar in structure to the antigen-receptor molecules on the original B cell's surface. An antibody can combine with the antigen on the pathogen and destroy the pathogen. A plasma cell is an antibody factory, as evidenced by its characteristically huge Golgi apparatus. At the peak of an infection, a plasma cell may produce and secrete 2,000 antibody molecules per second! Body fluids carry antibodies, which then react in various ways to destroy specific antigens or antigen-bearing particles. This antibody-mediated immune response is called the humoral immune response ("humoral" refers to body fluids). Other members of the activated B cell's clone differentiate further into memory B cells. Like memory T cells, these memory B cells respond rapidly to subsequent exposure to a specific antigen.

Lymphatic Pathways

- begin as lymphatic capillaries (closed-ended vessels) that merge to form lymphatic vessels. These, in turn, lead to even larger trunks and ducts that unite with the veins in the thorax. Lymphatic vessels transport excess fluid away from the interstitial spaces in most tissues and return it to the bloodstream. Without the lymphatic system, this fluid would accumulate in tissue spaces. Special lymphatic capillaries (lacteals) in the lining of the small intestine absorb digested fats, then transport the fats to the venous circulation.

Lymphatic Trunks

- drain lymph from the lymphatic vessels, are named for the regions they serve. For example, the lumbar trunk drains lymph from the lower limbs, lower abdominal wall, and pelvic organs; the intestinal trunk drains the abdominal viscera; the intercostal and bronchomediastinal trunks drain lymph from portions of the thorax; the subclavian trunk drains the upper limb; the jugular trunk drains portions of the neck and head. These lymphatic trunks then join one of two collecting ducts—the thoracic duct or the right lymphatic duct.

Inflammation

- is a reaction that produces localized redness, swelling, heat, and pain. The redness is a result of blood vessel dilation that increases blood flow and volume in affected tissues (hyperemia). This effect, coupled with an increase in permeability of nearby capillaries and subsequent leakage of protein-rich fluid into tissue spaces, swells tissues (edema). The heat comes as blood enters from deeper body parts, which are warmer than the surface. Pain results from stimulation of nearby pain receptors. Most inflammation is a tissue response to pathogen invasion, but physical factors (heat, ultraviolet light) or chemical factors (acids, bases) can also cause it. White blood cells accumulate at the sites of inflammation, where some of them help control pathogens by phagocytosis. Neutrophils are the first to arrive at the site, followed by monocytes. Monocytes pass through capillary walls (diapedesis), becoming macrophages that remove pathogens from surrounding tissues. In bacterial infections, the resulting mass of white blood cells, bacterial cells, and damaged tissue may form a thick fluid called pus. Fluids called exudates also collect in inflamed tissues. These fluids contain fibrinogen and other clotting factors that may stimulate formation of a network of fibrin threads in the affected region. Later, fibroblasts arrive and secrete fibers around the area, enclosing it in a sac of connective tissue. This walling off of the infected area helps inhibit the spread of pathogens and toxins to adjacent tissues. Once an infection is controlled, phagocytic cells remove dead cells and other debris from the site of inflammation. Cell division replaces lost cells.

Lymphatic Capillaries

- microscopic, closed-ended tubes. They extend into the interstitial spaces, forming complex networks that parallel the blood capillaries throughout the body The walls of lymphatic capillaries, like those of blood capillaries, are formed from a single layer of squamous epithelial cells called endothelium. These thin walls allow tissue fluid (interstitial fluid) from the interstitial space to enter the lymphatic capillaries. Once inside lymphatic capillaries, the fluid is called lymph. The lymphatic capillaries merge into lymph vessels.

Right Lymphatic Duct

- originates in the right thorax at the union of the right jugular, right subclavian, and right bronchomediastinal trunks. The right lymphatic duct empties into the right subclavian vein near its junction with the right jugular vein. Lymph from the lower body regions, the left upper limb, and the left side of the head and neck enters the thoracic duct; lymph from the right side of the head and neck, the right upper limb, and the right thorax enters the right lymphatic duct. After lymph leaves the two collecting ducts, it enters the venous system and becomes part of the plasma just before blood returns to the right atrium. Drains into the Right Lymphatic Duct: - Right upper limb - Right side of thorax - Right head/neck 1. Lymphatic Capillary 2. Afferent Lymphatic Vessel 3. Lymph Node 4. Efferent Lymphatic Vessel 5. Collecting Duct 6. Subclavian Vein

Species Resistance

- refers to the fact that a species may be resistant to diseases that affect other species. The basis of species resistance is that the cells of a resistant species do not have receptors for the pathogen, or its tissues do not provide the temperature or chemical environment that a particular pathogen requires. For example, humans contract measles, but other animal species do not. Similarly, humans are resistant to canine distemper that affects dogs.

Phagocytosis

- removes foreign particles from the lymph as it moves from the interstitial spaces to the bloodstream. Phagocytes in the blood vessels and in the tissues of the spleen, liver, or bone marrow usually remove particles that reach the blood. that the most active phagocytic cells of the blood are neutrophils and monocytes. Chemicals released from injured tissues attract these cells by chemotaxis. Neutrophils engulf and digest smaller particles; monocytes phagocytize larger ones. Monocytes that leave the blood differentiate to become macrophages. These large cells may be free or fixed in various tissues, including the lymph nodes, spleen, liver, and lungs, or attached to the inner walls of blood and lymphatic vessels. A macrophage can engulf up to 100 bacteria, compared to the twenty or so bacteria that a neutrophil can engulf. Monocytes and macrophages constitute the mononuclear phagocytic system (reticuloendothelial system).

Fever

A fever is a nonspecific defense that offers powerful protection. A fever begins as a viral or bacterial infection stimulates lymphocytes to proliferate, producing cells that secrete a substance called interleukin-1 (IL-1), more colorfully known as endogenous pyrogen ("fire maker from within"). IL-1 raises the thermoregulatory set point in the brain's hypothalamus to maintain a higher body temperature. Fever indirectly counters microbial growth because higher body temperature causes the liver and spleen to sequester iron, which reduces the level of iron in the blood. Because bacteria and fungi require iron for normal metabolism, their growth and reproduction in a fever-ridden body slow and may cease. Also, phagocytic cells attack more vigorously when the temperature rises. For these reasons, low-grade fever of short duration may be a natural response to infection, not a treated symptom.

Immunoglobulin E (IgE)

Immunoglobulin E (IgE) appears in exocrine secretions with IgA. It is associated with allergic responses

inflamm

to set on fire: inflammation—localized redness, heat, swelling, and pain in the tissues.