Microbiology 7 - The Lines of Defense PDF

Microbiology The Lines of Defense Microbiology| The Lines of Defense Contents : The First Line of Defense 3 The Second Line of Defense 11 The Third Line of Defense 33 Microbiology| The First Line of Defense The human body is like a fort with an outer barrier that wards off potential invaders. One co...

Microbiology The Lines of Defense Microbiology| The Lines of Defense Contents : The First Line of Defense 3 The Second Line of Defense 11 The Third Line of Defense 33 Microbiology| The First Line of Defense The human body is like a fort with an outer barrier that wards off potential invaders. One component of this outer wall is the skin, a protective layer that repels many potentially harmful microorganisms. The skin consists of a relatively thick and impermeable layer of epidermal cells, the epidermis, overlying the rich vascular layer, the dermis. Microbiology| The First Line of Defense Epidermal cells are produced by cell division in the base of the epidermis. As the basal cells proliferate, they move outward, become flattened, and die. The dead cells contain a protein called keratin (CARE-ah-tin). Keratin forms a fairly waterproof protective layer that not only reduces moisture loss but also protects underlying tissues from microorganisms. Microbiology| The First Line of Defense The cells of the epidermis are joined by special structures known as tight junctions, which impede water loss and microbial penetration. Although the skin protects us from infections, the human body contains three passageways that penetrate into its interior, providing a fairly direct route for microbes to enter the body. Microbiology| The First Line of Defense The passages into the interior are the respiratory, digestive, and urinary tracts. All three of them are protected by epithelial linings, although they are not as thick and impenetrable as the skin. As a result, they are more easily penetrated by microorganisms. Microbiology| The First Line of Defense The body's first line of defense also consists of chemical barriers. The skin, for example, which serves as a physical barrier, also produces slightly acidic secretions that impair bacterial growth. Acids are delivered to the skin in oil (sebum) from sebaceous glands and perspiration from sweat glands. Microbiology| The First Line of Defense Sweat also contains a natural antibiotic known as dermicidin. It's a peptide that kills microorganisms and helps protect us from potentially harmful bacteria. In addition, sebum (skin oil) also contains certain fatty acids with antibacterial properties. The stomach lining produces hydrochloric acid, which destroys many ingested bacteria. Microbiology| The First Line of Defense Tears and saliva contain an enzyme called lysozyme (LIE-so-zime), which dissolves the cell walls of bacteria, killing them. Cells in the lining of the respiratory tract produce mucus, which has antimicrobial properties. The skin, the epithelial linings, and chemical barriers are all nonspecific. Like a castle wall, they operate indiscriminately against all invaders. The Second Line of Defense Microbiology| The Second Line of Defense The first line of defense is not impenetrable. Even tiny breaks in the skin or in the lining of the respiratory, digestive, or urinary tracts may permit potentially harmful microorganisms to enter the body. Fortunately, a second line of defense exists. It involves a host of chemicals and cellular agents. This section discusses four components of the second line of defense: the inflammatory response, pyrogens, interferons, and complement. Microbiology| The Second Line of Defense The Inflammatory Response : Damage to body tissues triggers a series of reactions collectively referred to as an inflammatory response. The inflammatory response is characterized by redness, swelling, and pain. (The word inflammatory refers to the heat given off by a wound.) The inflammatory response is a kind of chemical and biological warfare waged against bacteria, viruses, and other microorganisms. The inflammatory response begins with the release of a variety of chemical substances by the injured tissue. Microbiology| The Second Line of Defense Some chemicals attract macrophages that reside in body tissues and neutrophils in the blood. These cells engulf (phagocytize) bacteria that enter a wound. Soon after these cells begin to work, a yellowish fluid begins to exude from the wound. Called pus, it contains dead white blood cells (mostly neutrophils), microorganisms, and cellular debris that accumulate at the wound site. Microbiology| The Second Line of Defense The Inflammatory Response : Microbiology| The Second Line of Defense Another chemical substance released by injured tissues is histamine. Histamine stimulates the arterioles in the injured tissue to dilate, causing the capillary networks to swell with blood. The increase in the flow of blood through an injured tissue is responsible for the heat and redness around a cut or abrasion. Heat, in turn, increases the metabolic rate of cells in the injured area and accelerates healing. Microbiology| The Second Line of Defense Still other substances released by injured tissues increase the permeability of capillaries, which increases the flow of plasma into a wounded region. Plasma carries oxygen and nutrients that aid in healing. It also carries the molecules necessary for blood clotting. The clotting mechanism walls off injured vessels and reduces blood loss. Plasma leaking into injured tissues causes swelling, which stimulates pain receptors in the area. Microbiology| The Second Line of Defense Pain receptors send nerve impulses to the brain. Pain also results from chemical toxins released by bacteria and from chemicals released by injured cells. One important pain-causing chemical is prostaglandin. Aspirin and other mild painkillers work by inhibiting the synthesis and release of prostaglandins. Inflammation even comes with its own cleanup crew in the form of late-arriving monocytes. These cells phagocytize dead cells, cell fragments, dead bacteria, and viruses. Microbiology| The Second Line of Defense Pyrogens, Interferons, and Complement : Pyrogens are molecules released primarily by macrophages that have been exposed to bacteria. Pyrogens travel to the hypothalamus (a region of the brain). In the hypothalamus is a group of nerve cells that controls the body's temperature, in much the same way that a thermostat regulates the temperature of a room. Microbiology| The Second Line of Defense Pyrogens turn the thermostat up, increasing body temperature and producing a fever. Mild fevers cause the spleen and liver to remove additional iron from the blood. Because many disease-causing bacteria require iron to reproduce, fever helps reduce their replication. Fever also increases metabolism, which facilitates healing and accelerates cellular defense mechanisms such as phagocytosis. Microbiology| The Second Line of Defense Important as it is, fever can also be debilitating, and a severe fever (over 105°F [41°C]) is potentially life threatening because it begins to denature (coagulate) vital body proteins, especially enzymes needed for biochemical reactions in body cells. Another chemical safeguard is a group of small proteins known as the interferons. Some interferons are released from cells infected by viruses. Microbiology| The Second Line of Defense They diffuse away from the site of production through the interstitial tissue and bind to receptors on the plasma membranes of noninfected body cells. How Interferon Works Interferon protects cells from viral infection Microbiology| The Second Line of Defense The binding of interferon to uninfected cells, in turn, triggers the synthesis of cellular enzymes that inhibit viral replication. Thus, when viruses enter the previously uninfected cells, they cannot replicate and spread. Microbiology| The Second Line of Defense Interferons do not protect cells already infected by a virus; they help stop the spread of viruses from one cell to another. In essence, the production and release of interferon are the dying cell's last act to protect other cells of the body. Interferons help to slow the spread of viruses, while the immune response attacks and destroys the viruses outside the cells. Microbiology| The Second Line of Defense Currently, about a dozen interferons are used to treat diseases. Some are used, for instance, to treat patients with viral hepatitis, known as hepatitis C. Other types of interferons are used to combat certain forms of cancer, for example, some forms of leukemia, Kaposi's sarcoma (a cancer that is commonly found in AIDS patients), and malignant melanoma, a fairly aggressive form of skin cancer. Still others are used to treat the neurological disorder multiple sclerosis. Microbiology| The Second Line of Defense Interferons used to treat cancer boost the immune system response by regulating several genes that suppress the production of certain cellular proteins that stimulate growth, which reduces the growth of cancer cells. Interferons are made by companies using a genetic engineering technique known as recombinant DNA technology. It's basically a way to mass produce proteins in the laboratory. Microbiology| The Second Line of Defense Like many medical treatments, interferons have undesirable side effects like fever, chills, headache, and muscle aches. They vary from mild to severe and occur in about 50% of all patients receiving them. These side effects typically decrease after repeated injections and can also be countered by pain medications (acetaminophen) and antihistamines. Microbiology| The Second Line of Defense Severe side effects like liver failure have also been observed in patients receiving high doses of interferon. Another group of chemical agents that fight infection are the complement proteins. These blood proteins form the complement system, so named because it complements the action of antibodies. Microbiology| The Second Line of Defense The details of the complement system are very complex. Complement proteins circulate in the blood in an inactive state. When foreign cells such as bacteria invade the body, the complement protein is activated. This triggers a chain reaction in which one complement protein activates the next. Microbiology| The Second Line of Defense Five proteins in the complement system join to form a large protein complex, known as the membraneattack complex. The membrane-attack complex embeds in the plasma membrane of bacteria, creating an opening into which water flows. The influx of water causes bacterial cells to swell, burst, and die. The Membrane-Attack Complex Microbiology| The Second Line of Defense Five complement proteins combine and embed in a cell's membrane, causing it to leak, swell, and burst. Several of the activated complement proteins also function on their own and are part of the inflammatory response. Some of them, for example, stimulate the dilation of blood vessels in an infected area, Others increase the permeability of the blood vessels, allowing white blood cells and nutrient-rich plasma to pass more readily into an infected zone. Microbiology| The Second Line of Defense Certain complement proteins may also act as chemical attractants, drawing macrophages, monocytes, and neutrophils to the site of infection, where they phagocytize foreign cells. Yet another complement protein (C3b) binds to microorganisms, forming a rough coat on the intruders that facilitates their phagocytosis. The Third Line of Defense Microbiology| The Third Line of Defense The immune system is the third line of defense. Unlike the respiratory or digestive systems, the immune system is rather diffuse—spread out and indistinct. Lymphocytes, for example, are part of the immune system. They circulate in the blood and lymph and also take up residence in the lymphoid organs such as the spleen, thymus, lymph nodes, tonsils, and body tissues. All of these are part of the immune system. Microbiology| The Third Line of Defense The immune system, like the first and second lines of defense, is an important homeostatic mechanism that eliminates foreign organisms including bacteria, viruses, single-celled fungi, and many parasites. However, the cells of the immune system selectively target foreign substances and foreign organisms. As a result, the immune system is said to be specific. Microbiology| The Third Line of Defense The immune system comes into play when foreign organisms penetrate the outer defenses of the body the first and second lines of defense. The immune system also helps prevent the emergence and development of cancer. As in other body systems, optimal immune system function depends on optimum health, excellent nutrition, exercise, and sufficient sleep. Microbiology| The Third Line of Defense For example, studies show that eating foods rich in micronutrients, especially phytochemicals, dramatically reduces one's chances of developing common illnesses like colds and flus, or the duration of these illnesses. It also reduces severe illnesses such as autoimmune disease, heart disease, and cancer. Microbiology| The Third Line of Defense One of the chief functions of the immune system is to identify what belongs in the body and what does not. Once a foreign substance has been detected, the immune system mounts an attack to eliminate it. Therefore, like all homeostatic systems, the immune system requires receptors to detect changes and effectors to bring about appropriate responses. In the immune system, the lymphocytes serve both functions. Microbiology| The Third Line of Defense Antigens : The immune response is triggered by a variety of foreign cells, among them, bacteria, viruses, fungi, parasites, and cells derived from skin grafts or blood transfusion from genetically dissimilar individuals. The common denominator in all of these cases is that all these cells and even viruses to which our immune systems respond contain large molecules in their outermost coats or membranes, proteins and polysaccharides. These molecules are called antigens, which is an abbreviation for antibody-generating substances. Microbiology| The Third Line of Defense When a bacterium or virus enters your body, your immune system recognizes that it doesn't belong there that is, it is a foreigner. It's the unique assemblage of proteins and carbohydrates in the outer membranes of the foreign organisms that your immune system recognizes as not belonging. These include viruses, bacteria, single-celled fungi, and parasites such as the protozoan that causes malaria. Microbiology| The Third Line of Defense Cells transplanted from one person to another also elicit an immune response. That's because each individual's cells contain a unique “cellular fingerprint” resulting from the unique array of plasma membrane glycoproteins. The immune system is activated by these antigens on the foreign cells. Cancer cells that arise within your body also present a slightly different chemical fingerprint, making them essentially foreign cells within our own bodies to which the immune system responds. Microbiology| The Third Line of Defense Although cancer cells evoke an immune response, it is often not sufficient to stop the disease. As a rule, small molecules generally do not elicit an immune reaction. In some individuals, however, small, nonantigenic molecules such as formaldehyde, penicillin, and the poison ivy toxin bind to naturally occurring proteins in the body, forming complexes. These large complexes are unique compounds that are foreign to the body and can cause an immune response.