Chapter 12: Lymphatic System and Body Defenses



Chapter 12: Lymphatic System and Body Defenses

Lecture Notes taken from:

Marieb,E.N. 2009. Essentials of Human Anatomy and Physiology. PBC

Lymphatic System: Not all blood plasma that leaks out of the capillaries to nurture body tissues is returned to the blood on the venule side of the capillary bed. In order to maintain the blood volume, however, that lost fluid must be returned somehow. One primary function of the lymphatic system is to return that fluid to the blood. As interstitial fluid (formerly blood plasma) bathes tissues, it also captures bits of broken down cells and often washes away pathogens (if present). So, along with maintaining blood volume, the lymphatic system also filters out debris before the fluid is returned to the blood AND monitors the body for pathogens as part of the immune system.

The Lymphatic System is composed of: Lymphatic capillaries – first level of entry for excess body fluids, minivalves make certain that lymph only enters the capillaries (i.e. it can’t leak out once inside the lymph capillaries), found weaving throughout the capillary beds and body tissues; Lymphatic vessels – carry lymph toward the heart and in much the same way as veins; valves inside of lymph vessels prevent back flow of lymph, skeletal muscle contractions ‘milk’ the lymph toward the heart, smooth muscle around the largest lymph vessels also help propel lymph; Lymph nodes – organs that contain defense cells (macrophages and lymphocytes) that actually perform the filtering and monitoring functions of the lymphatic system, receive lymph from several afferent lymph vessels and send out filtered lymph through efferent lymph vessels on the way toward the heart.

Other lymph organs do not actually filter lymph, but do filter other body fluids to monitor pathogens and rid the body fluids of impurities. These include: spleen – filters and stores blood, recycles old blood cells; thymus – programs some of the lymphocytes that actually inhabit the lymph nodes and organs; tonsils – filters mucus of the throat; Peyer’s patches – filters mucus of the intestinal lining.

Body Defenses: Your body is constantly under attack by pathogens (harmful or disease carrying microorganisms) that include viruses, bacteria, and fungi among others. Our body protects us with both an innate (a.k.a. non-specific) defense system that responds immediately to a variety of invaders and an adaptive (a.k.a. specific) defense system that recognizes and attacks very specific pathogens. Together, they make up 3 ‘lines of defense’. The first 2 lines of defense are provided by nonspecific defense mechanisms.

The First Line of Nonspecific Defense: This includes the skin, mucus membranes (lining of all body cavities open to exterior), and their secretion. These defense mechanisms prevent pathogens from entering the body in the first place. The keratinized surface of the cutaneous membrane and the outer layers of mucous membranes are strong physical barriers to pathogens if intact. Furthermore, most body surfaces secrete chemicals that also thwart pathogens. Examples include: acidic pH of skin, stomach, vaginal secretions that kill bacteria; antibiotics in sebum; lysozymes in saliva and lacrimal fluid; digestive enzymes of the intestinal tract; and sticky mucus that traps pathogens in the respiratory and digestive tracts.

The Second Line of Nonspecific Defense: This includes internal cells and chemicals of the body that constantly patrol and circulate the body in order to detect any ‘nonself’ invaders. Cellular components on this second line of defense include phagocytic cells (cells that consume other cells and debris; phago = eat) like macrophages and neutrophils AND non-phagocytic cells known as natural killer (NK) cells that release toxins to kill cells or organisms detected as nonself. This second line of defense also includes a coordinated set of chemical and cellular responses known as the inflammatory response. The inflammatory response is triggered by tissue damage. Such damage stimulates the release of chemicals (like histamine and others) from the damaged cells that act as local hormonal messengers to dilate blood vessels (increases blood flow to the area to deliver WBCs and nourish local cells), attract WBCs, and form clots to seal off the wounded area. This response effectively isolates any potential pathogens to keep them from spreading throughout the body, helps kill any of those pathogens, and clears the area of debris so that repair of the tissues can begin. Strictly chemical components of the nonspecific defense system include: interferon = antimicrobial protein released by infected cells to warn nearby cells of impending infection, interferon is active in many antimicrobial pathways including ones that prevent viruses from replicating; complement proteins = proteins that circulate in the blood and bind to nonself cells, binding of complement proteins punctures the cell membranes of pathogens to kill them. Fever is also considered part of the internal nonspecific defense system. Pyrogens secreted by WBCs battling pathogens reset the internal thermometer of the hypothalamus to a higher level. The higher body temperature increases the metabolism of the defending cells and stimulates the kidneys and liver to hold onto certain nutrients required by bacteria.

The Third Line of Defense = Adaptive Defense System: Unlike the innate (nonspecific) system, the adaptive defense system responds to specific pathogens based on particular antigens (molecules, usu. proteins) found only on the surface of that pathogen. Upon identification of a particular pathogen, the adaptive response will provide protection throughout the body, i.e. protection is systemic and not just localized. Once the pathogen is overcome from the first infection, the adaptive system is primed for a quick and strong response in case of a second attack; this is referred to as memory.

2 divisions of Adaptive Defenses: 1. Humoral immunity = antibody-mediated immunity requires the production of huge proteins called antibodies that immobilize or kill pathogens by binding to their nonself antigens.

2. Cellular immunity = cell-mediated immunity requires the activity of defense cells that bind to the pathogens and release toxins in order to kill them. Both types still recognize the pathogens by particular antigens.

3 major cell groups are involved in the adaptive defense system: B lymphocytes –lymphocytes that originate and become immunocompetent (capable of binding to an antigen) in bone marrow (B = bone); T lymphocytes also originate in the bone marrow, but they migrate to the Thymus before becoming immunocompetent (T = thymus); and macrophages are responsible for activating T cells. As the term lymphocyte suggest, these cells are found primarily in the lymph nodes and organs where they await pathogens (antigens) delivered by the lymph vessels.

Humoral immunity is primarily a function of the B lymphocytes. After B-lymphocytes mature, they migrate to lymph nodes and monitor the lymph for antigens. Each B-lymphocyte can recognize only 1 free antigen, and if that antigen is never present in the lymph, the B-cell will remain dormant. However, if a B-cell does bind to an antigen, it quickly divides into an army of clones. That army of clones then starts making huge quantities of antibodies that are released into the lymph and blood stream to thwart the invader. A small subset of these same B-cells becomes memory B-cells. If the pathogen infects the body a second time, the memory B cells respond much more quickly and with greater antibody production than that stimulated by the original (primary) infection. The battle during a primary infection takes about 10 days to peak. With memory cells and a secondary infection, the defenses peak in 2-3 days and the number of antibodies produced is many times greater. It is important to note that ‘free’ antigens = relatively small particles like viruses or toxins released by bacteria. To make antibodies that attack antigens are larger cells, the B-cells must be activated by helper T-cells (see cellular immunity below).

Summary of Humoral Response to Free Antigens:

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Cellular immunity is primarily driven by T cells. For some reason, T-cells cannot bind directly to an antigen. However, if a macrophage consumes a pathogen as part of the innate response, it will ‘present’ antigens of the digested pathogen to T-cells (usually in the lymph nodes). When a T-cell binds to a presented antigen, it divides into an army of clones just like the B-cells. Instead of producing antibodies, however, many of the T-cells become cytotoxic (killer) T-cells that bind directly to pathenogenic cells (including our own cells infected by viruses and cancer cells) and kill them like natural killer cells. Other T-cells become helper T-cells or memory T-cells (just like memory B cells). Among other things, helper T-cells can induce antibody production by B cells when the pathogens are large cells (either cellular pathogens or infected cells). See summary diagram below:

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