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An epidemiological study in which an investigator tests the hypothesis that a particular treatment will be effective in controlling a disease for which no accepted cure is available is known as a(n) study medications similar to lyrica buy generic baycip 500 mg on-line. Because most pathogens cannot survive for extended periin ods of time outside the body, they must persist within order to maintain their ability to infect humans. Salmonella typhi has the ability to persist within the gallbladder of humans while causing no clinical symptoms. The infected individual is still contagious, however, and would be considered a(n): (a) Pathogen (b) Nuisance (c) Endemic (d) Carrier (e) Bacteriophage 10. A disease in which a person contracts rabies virus after interaction with an infected raccoon would be known as a disease. The incidence rate can indicate whether there is an increase or decrease in the spread of a disease while the prevalence rate measures how seriously or long the disease is affecting a population. Match the following terms with the most appropriate description: - Epidemic disease - - Sporadic disease - - Common-source - outbreak - Pandemic disease - (a) Spread from local to global incidence (b) Occasional disease incidence, not spreading (c) Significantly increasing incidence, and spreading (d) A cluster of related infections with limited spread (e) Relatively low, steady incidence, but continually present - Endemic disease - 6. Match the following modes of disease transmission to their object(s): - Fomites - - Bar soap - - Kissing - - Speaking - - Hand shaking - - Food - - Housefly - - Mother breast-feeding - her infant - Anopheles mosquito - - Stepping on a rusty nail - 13. Hospital-acquired infections are called: (a) Notifiable diseases (b) Nosocomial infections (c) Pathogens (d) Staphylococcal infections (e) Transient infections (a) Horizontal (b) Vertical (c) Indirect contact (d) Vehicle (e) Biological vector (f) Droplet (g) Mechanical vector 17. Fill in the blanks: Health care workers apply precautions when treating any patient, and additional precautions when treating patients with highly communicable diseases. The extensive use of antibiotics as well as gene transfer has led to more virulent, antibiotic-resistant strains of all of the following organisms except: (a) Streptococcus pneumoniae (b) Staphylococcus aureus (c) Enterococci (d) Influenza (e) Escherichia coli 19. Suppose that the health department of city A mounts a successful campaign to get children immunized against measles. Now suppose that city B, the same size as city A, has not carried out a successful measles vaccination program. Of the 10,000 children in city B, 5,000 had measles when the disease last struck the population. Go to the companion website to fine-tune your understanding of the chapter concepts and learn more about the topics below. The Italian writer Boccaccio said its victims often "ate lunch with their friends and dinner with their ancestors in paradise. At its peak in 1918, the global flu epidemic was responsible for 11,000 deaths in 1 month. This mucus is important to us as a protective barrier, preventing pathogens from reaching the gut and the nasal tissues. Once at the tissue surface, the pathogens could invade, colonize, and cause infection. So, the mucus has long been recognized as part of our innate immune system, for its ability to trap undesirable microbes and particles. What is newly found is the abundance of bacteriophages Bacteriophage attacking an infectious bacterium. Well, in 2013 a big discovery was made-a new in the mucus that are busy killing off bacteria. Bacteria also accumulate in the mucus when their flagella prove useless at propelling them through it because of its great density. The phages stick specifically to the mucus by adhering to various branched sugar molecules characteristic of mucus. The phages multiply within the bacteria, killing them, generating more predatory phages, and keeping the bacterial population under control. Without "our friends" the bacteriophages, life would be just, one infection after the next-until a fatal one occurs. In Chapters 14 and 15 we considered how infectious agents enter and damage the body and how they leave the body and spread through populations. In the next three chapters we consider how the body resists invasion by infectious agents. As the nonspecific defenses were studied, it became apparent that they involved very specific interactions but did not require a previous exposure to be active, hence the term innate defense. Then we will look at the innate defense mechanisms in more detail to see how they function in protecting the body against infectious agents. The answer is that our bodies have defenses for resisting the attack of many dangerous organisms. Only when our resistance fails do we become susceptible to infection by pathogens. Viruses and pathogenic bacteria have molecules in or on them which serve as antigens. Adaptive defenses then respond to these antigens by pro ducing protein antibodies. The human body is capable of making millions of different antibodies, each effective against a particular antigen. These antibody and cellular responses are more effective against succeeding inva sions by the same pathogen than against initial invasions thanks to memory cells. Often such defenses perform their function before adap tive body defense mechanisms are activated. Physical barriers, such as the skin and mucous mem branes and the chemicals they secrete. Chemical barriers, including antimicrobial substances in body fluids such as saliva, mucus, gastric juices, and the iron limitation mechanisms. Cellular defenses, consisting of certain cells that engulf (phagocytize) invading microorganisms. Inflammation, the reddening, swelling, and tempera ture increases in tissues at sites of infection. Fever, the elevation of body temperature to kill invad ing agents and/or inactivate their toxic products. Molecular defenses, such as interferon and complement, that destroy or impede invading microbes. The physical and certain chemical barriers operate to prevent pathogens from entering the body. The other in nate defenses (cellular defenses, inflammation, fever, and molecular defenses) act to destroy pathogens or inactivate the toxic products that have gained entry or to prevent the pathogens from damaging additional tissues. Their beneficial effects come from their ability to permanently block an enzyme that promotes inflammation, pain, and fevers. Unfortunately, the drugs are even more effective at permanently inhibiting a related enzyme that is necessary for the health of the stomach and kidneys. Underactivity will leave the host open to overwhelming infection (sepsis) leading to death. Vomiting and diarrhea similarly act to flush harmful microbes and their chemical products from the di gestive tract. Likewise, urinary flow is important in re moving microbes that enter the urinary tract. Urinary tract infections are especially common among those unable to empty their bladder completely or frequently enough. These two physical barriers are made of cells that line the body surfaces and secrete chemicals, making the surfaces hard to penetrate and inhospitable to pathogens. The skin, for example, not only is exposed directly to microorgan isms and toxic substances but also is subject to objects that touch, abrade, and tear it. Cuts, scratches, insect and animal bites, burns, and other wounds can disrupt the continuity of the skin and make it vulnerable to infection. Besides the skin, a mucous membrane, or mucosa, cov ers those tissues and organs of the body cavity that are ex posed to the exterior. Mucous membranes, therefore, are another physical barrier that makes it difficult for pathogens to invade internal body systems. The hairs and mucus of the nasal and respiratory sys tem present mechanical barriers to invading microbes. Both sweat and the sebum produced by sebaceous glands in the skin produce secre tions with an acid pH that inhibits the growth of many bacteria. The very acidic pH of the stomach is a major innate defense against intestinal pathogens. Lysozyme, an enzyme present in tears, saliva, and mucus, cleaves the covalent linkage between the sugars in peptidogly can; hence, Grampositive bacteria are particularly sus ceptible to killing by this enzyme (Chapter 19, p.

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Should a vaccine be developed medications that interact with grapefruit best 500mg baycip, a means to mount a costly, massive vaccination program would be needed. Because available vaccines, such as those for measles and mumps, have not been administered to many of the children in the region endemic to African sleeping sickness, the prospect for mass immunization is not bright. The disease occurs sporadically in the southern United States and is endemic in Mexico, Central America, and much of South America. However, there are other species of reduviids living in most of the United States. A greater worry is global warming, which could extend the range of southern species well into the United States. Reduviid bugs often bite near the eyes; they defecate as they bite, depositing infectious parasites on the skin. Humans almost automatically rub such a bug bite and thereby transfer parasites to the eyes or the bite wound. After 1 to 2 weeks, the parasites have made their way to lymph nodes, where they repeatedly divide and form aggregates called pseudocysts. These parasites enter cells either by invasion or by phagocytosis and can damage lymphatic tissues, all kinds of muscle, and especially supporting tissues around nerve ganglia. Destruction of nerve ganglia in the heart accounts for nearly three-fourths of deaths from heart disease among young adults in endemic areas. The acute disease, which is most common in children under 2, is characterized by severe anemia, muscle pain, and nervous disorders. In especially virulent acute disease, death can occur in 3 to 4 weeks, but many patients recover after several months of less virulent disease. The chronic disease, which is seen mainly in adults, probably arises from a childhood infection. It is a mild disease and is sometimes asymptomatic but often causes enlargement of various organs. In the digestive tract, it slows or stops muscle contractions due to death of up to 85% of the neurons in the esophagus and 50% of those in the colon; in the heart, it can cause irregular heartbeat and accumulation of fluid around the heart; in the central nervous system, it can cause paralysis by destroying motor centers. In some parts of the world, untested blood used for transfusion also spreads the disease. Small animals such as guinea pigs and mice can be inoculated with blood from patients and observed for disease symptoms. Xenos is Greek for activate T cells of the "strange" or "foreign"; in this context immune system; in it refers to the use of an organism difthe presence of Tryferent from a human. Despite several ways to diagnose the disease, no effective treatment is available. Drugs used to treat other trypanosome infections are of no use because they fail to reach the parasites inside cells. Work is under way to develop new drugs and a vaccine, but until these are available, control of the reduviid vectors is the only means of reducing misery from this disease. Treating homes with insecticides offers some protection, but the bugs crawl into crevices in walls and thatched roofs and are difficult to eradicate. The agents and characteristics of the diseases discussed in this chapter are summarized in Table 24. Compare the advantages and disadvantages of live (oral) versus killed polio vaccines. Many patients are asymptomatic for years after infection, and no diagnostic test is available to identify such patients. Botulism is acquired by ingesting foods that contain a potent preformed neurotoxin and is treated with polyvalent antitoxin. Prior to the development of vaccines in the 1950s, poliomyelitis was a common and dreaded disease. Treatment merely alleviates symptoms and includes the use of an iron lung to maintain breathing when respiratory muscles are paralyzed. Listeriosis can be transmitted by improperly processed dairy products and can cross the placenta. Both an injectable and an oral polio vaccine are available; each has advantages and disadvantages. Antibiotics are effective if given early in an infection, and surgery can be used later unless the abscess is in a vital area of the brain. Prion Diseases of the Nervous System Following a long delay after inoculation, brain cells die, causing a spongiform appearance of the tissue. Viral Diseases of the Brain and Meninges Symptoms begin with spasms, which rapidly worsen to collapse, followed by death. It is difficult to control because of the large number of small mammals that serve as reservoirs. It can be prevented by immunizing pets and people at risk and by avoidance of contact with wild animals by those who are not immunized. Parasitic Diseases of the Nervous System African sleeping sickness occurs in equatorial Africa and is transmitted by the tsetse fly. It is diagnosed by finding parasites in the blood and can be treated with pentamidine and other drugs. Encephalitis is transmitted by mosquitoes, often from horses, and can sometimes be diagnosed by culturing blood or spinal fluid in cells or mice. While wrapping the dog, Harry was covered in its blood, and was bitten and scratched several times. The dog died en route to the animal hospital, so Harry disposed of it in a convenient dumpster. He became reclusive, insisted on closing drapes over the window, refused to have lights on in the room, stopped eating, was very thirsty but refused to drink water and other liquids, laid in bed, and made strange noises. They did, and had to take him away forcibly to the hospital, where he died a few days later. Think carefully before you stop to help an injured animal-this case is based on a true story. Why do you think lipid-soluble antibiotics like chloramphenicol and tetracycline work better in combating bacterial meningitis Rabies is among the few diseases where people can be immunized after the pathogen has already entered their body. Newborns suffer from meningitis caused by bacterial species that differ from the ones that cause meningitis in older children primarily because: (a) Newborns are exposed to a different reservoir of infection during childbirth (b) They lack immunity (c) Their meninges are not well developed (d) Their vascular system is not well developed 2. Which of the following is a common cause of meningitis in nonimmunized young children A complication of infection with this organism (Waterhouse-Friderichsen syndrome) can occur if the organism becomes widely distributed in the body, leading to endotoxin shock and death. The organism is: (a) Haemophilus influenzae type A (b) Neisseria meningitidis (c) Haemophilus influenzae type B (d) Streptococcus pneumoniae (e) Listeria monocytogenes 5. Before development of a vaccine against this microbe, the disease it caused accounted for two-thirds of bacterial meningitis cases during the first year of life but is still the number one leading cause of mental retardation in patients who survive serious disease due to permanent central nervous system disorders. When microscopic analysis of cerebral spinal fluid reveals inflammatory cells, but no visible pathogens, the most likely diagnosis is: (a) Absence of infection (b) Bacterial infection (c) Viral infection (d) Fungal infection (e) Protozoan infection 7. Viral meningitis is usually always fatal if untreated, unlike bacterial meningitis, which is always self-limiting and non- fatal. The viruses that cause encephalitis are most likely to be: (a) Togaviruses (b) Enteroviruses (c) Mumps virus (d) Rabies virus (e) Hepatitis viruses 13. What do the following viral encephalitis diseases have in common: eastern equine encephalitis, western equine encephalitis, Venezuelan equine encephalitis, St. The only bacterium known to damage peripheral nerves is: (a) Naegleria fowleri (b) Mycobacterium leprae (c) Streptococcus pneumoniae (d) Neisseria meningitidis (e) Haemophilus influenzae 15. This toxin has been implicated as a cause of: (a) Meningitis (b) Myelitis (c) Motor neuron disease (d) Encephalitis (e) Spastic paralysis 17. The endospores of this organism are the most heat-resistant known; the organism is: (a) Clostridium tetani (b) Clostridium botulinum (c) Mycobacterium leprae (d) Streptococcus thermicos (e) Listeria monocytogenes 18. Which form of poliovirus vaccine is better at eliminating viruses in the gastrointestinal tract Creutzfeldt-Jakob disease: (a) Is a form of meningitis (b) Is slow to develop with a long period of recuperation before the patient is cured (c) Is a transmissible spongiform encephalopathy (d) Can only be transmitted by ingestion of prioncontaminated meat (e) Is always an inherited disease 23. Name a pathogen that can cause disease by infecting or affecting each part of the nervous system. Go to the compansion website to fine-tune your understanding of the chapter concepts and discover answers to the questions posed below.

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They inject an anticoagulant symptoms copd buy baycip 500 mg otc, and blood stains on the sheets from still oozing bites, are typical of infestations. Their life cycle includes 5 immature instar stages, each of which requires at least one blood meal before passing on to the next stage. Adults can go 4 1 months 2 without a blood meal, and may even last 18 months without one. Their preferred host is humans, but bats and birds that live in caves are also primary targets. Some people have no reaction to bed bug bites, but Microbiology Roadmap Visit the companion website for the Microbiology Roadmap with practice questions, current examples, and other tools to help you study, review, and master the key concepts of the chapter. The bugs release an unpleasant scent, which gives a room a characteristic odor that trained dogs can detect. PrinciPles of Parasitology 312 the Significance of Parasitism 312 Parasites in Relation to Their Hosts 312 Wolbachia 313 Protists 314 Characteristics of Protists 314 the Importance of Protists 314 Classification of Protists 315 fungi 321 Characteristics of Fungi 321 the Importance of Fungi 324 Classification of Fungi 325 HelmintHs 329 Characteristics of Helminths 329 Parasitic Helminths 330 artHroPods 337 Characteristics of Arthropods 337 Classification of Arthropods 337 In our survey of microbes, we have devoted significant attention to bacteria of the Domains Bacteria and Archaea, and to viruses. However, some members of the Domain Eukarya are also of interest to microbiologists, ecologists, and health scientists. The kingdoms Protista and Fungi contain large numbers of microscopic species, some of which supply food and antibiotics, and some of which cause disease. The kingdom Animalia contains helminths that cause disease and arthropods that cause or transmit diseases. Unless health scientists take a course in parasitology, their only opportunity to learn about helminths and arthropods is in conjunction with the study of microscopic infectious agents. The Significance of Parasitism Parasites have been a scourge throughout human history. In fact, even with modern technology to treat and control parasitic diseases, there are more parasitic infections than there are living humans. It has been estimated that among the 60 million people dying each year, fully one-fourth die of parasitic infections or their complications. As the world population increases, and the need for food with it, cultivation of such lands will become more important. In some inhabited regions, many people are near starvation and severely debilitated by parasites. Furthermore, parasitic infections in wild and domestic animals provide sources of human infection and cause debilitation and death among the animals, thus preventing the raising of cattle and other animals for food. Given the many human problems created by parasites, all citizens-and especially health scientists-need to understand the problems associated with the control and treatment of parasitic diseases. Although few people realize it, among all living forms, there are probably more parasitic than nonparasitic organisms. Many of these parasites are microscopic throughout their life cycle or at some stage of it. Historically, in the development of the science of biology, parasitology came to refer to the study of protozoa, helminths, and arthropods that live at the expense of other organisms. Strictly speaking, bacteria and viruses that live at the expense of their hosts also are parasites. The manner in which parasites affect their hosts differs in some respects from that described in earlier chapters for bacteria and viruses. This introduction 312 Parasites in relation to Their Hosts Parasites can be divided into ecto parasites, such as ticks and lice, which live on the surface of other organisms, and endoparasites, such as some protozoa A single tapeworm can live for 30 to 35 years. Its pear-shaped head is about I to 2 mm in diameter, and it may reach a length of 10 meters. Most parasites are obligate parasites: They must spend at least some of their life cycle in or on a host. Principles of Parasitology 313 A few parasites are facultative parasites: They normally are free-living, such as some soil fungi, but they can obtain nutrients from a host, as many fungi do when they cause skin infections. Hosts that are invaded by parasites usually lack effective defenses against them, so such diseases can be serious and sometimes fatal. Parasites are also categorized according to the duration of their association with their hosts. Permanent para sites, such as tapeworms, remain in or on a host once they have invaded it. Temporary parasites, such as many biting insects, feed on and then leave their hosts. Ticks that ordinarily attach to dogs or to wild animals sometimes attach to humans; the ticks are then accidental parasites. Some mosquitoes, which are temporary parasites, harbor the malaria parasite or other parasites. Such insects serve as vectors, or agents of transmission, of many human parasitic diseases. A vector in which the parasite goes through part of its life cycle is a biological vector. A mechanical vector is a vector in which the parasite does not go through any part of its life cycle during transit. Flies that carry parasite eggs, bacteria, or viruses from feces to human food are mechanical vectors. Hosts are classified as definitive hosts if they harbor a parasite while it reproduces sexually; they are said to be intermediate hosts if they harbor the parasite during some other developmental stages. The mosquito is the definitive host for the malaria parasite because that parasite reproduces sexually in the mosquito; the human is an intermediate host, even though humans suffer greater damage from the parasite. Reservoir hosts are infected organisms that make parasites available for transmission to other hosts. Reservoir hosts for human parasitic diseases typically are wild or domestic animals. Host specificity refers to the range of different hosts in which a parasite can mature. The worm that causes trichinosis can mature in almost any warm-blooded animal, but the parasite is most often acquired by humans from pigs through the consumption of inadequately cooked, contaminated pork. Over thousands of years of evolution, parasites tend to become less injurious to their hosts. Such an arrangement preserves the host so that the parasites are guaranteed a continuous supply of nutrients. Many parasites have one or more of the following mechanisms for evading host defense mechanisms: 1. Encystment, the formation of an outer covering that protects against unfavorable environmental condi- tions. These resistant cyst stages also sometimes provide a site for internal reorganization of the organism and cell division, help attach a parasite to a host, or serve to transmit a parasite from one host to another. Invading host cells, where the parasites are out of reach of host defense mechanisms. When parasites successfully evade host defenses, they can cause several kinds of damage. They cause open sores on the skin, destroy cells in tissues and organs, clog and damage blood vessels, and may even cause internal hemorrhages. Parasites that do not evade defense mechanisms sometimes trigger severe inflammatory and immunological reactions. For example, treatment to rid human hosts of some worm infections effectively kills the worms, but toxins from the dead worms cause more tissue damage than do the living parasites. The dog heartworm, Dirofilaria immitis, perforates the heart wall and leaves holes in the heart when the worms die and decay. Therefore, it is important for a veterinarian to test all dogs for the presence of heartworms before administering preventive heartworm medication. Parasitism, although an easy life once the parasite is established, is a hazardous existence during transfers from one host to another. For example, many parasites that leave the human body through feces die from desiccation (drying out) before they reach another host.

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Desensitization has been very successful against insect venoms and drug allergies such as allergies to penicillin medicine app discount baycip uk. Unfortunately, desensitization does not alleviate the signs and symptoms of many allergies, such as hay fever. In addition, the treatment itself can cause anaphylactic shock because the injections contain the very substance to which the patient is allergic. As we learn more about the properties of leukotrienes and IgE antibodies, perhaps this need can be met. Treatment of Allergies One approach to dealing with allergies is to avoid contact with the specific allergen. People with food allergies should not eat a food to which they have had a hypersensitivity reaction. Desensitization (hyposensitization) is the only currently available treatment intended to cure an allergy. The cells to which the antibodies are attached, as well as surrounding tissues, are damaged because of the resulting inflammatory response. Such shots may lead to tolerance, preventing B cells from maturing into plasma cells to make IgE antibodies. Exposure to the allergen also may activate those B cells that mature into the plasma cells that make IgG (blocking) antibody. Such IgG antibodies can bind to incoming allergen before it reaches the IgE molecules attached to mast cells. Complexing of allergen with these attached IgE molecules would cause the mast cells to degranulate and release histamine, so blocking this step is the key to preventing allergic response. Mechanism of Cytotoxic Reactions When an antigen on a plasma membrane is first recognized as foreign, B cells become sensitized and stand ready for antibody production upon a subsequent antigen exposure. During subsequent exposures with the surface antigen, antibodies bind to the antigen and activate complement. Transfusion Reactions Normal human red blood cells have genetically determined surface antigens (blood group systems) that form the basis for the different blood types. Research has shown that some people who lack the ability to produce IgE antibodies are prone to lung and sinus infections. Also, people who lack the ability to produce IgG or IgM antibodies often produce IgE antibodies to bacterial infections. These observations suggest that the IgE antibody may play a necessary role in immunity, besides causing allergies. The American biologist Margie Profet believes that IgE antibodies also are a backup system to protect against the ingestion of toxins. In the book Why We Get Sick: the New Science of Darwinian Medicine, the authors, physician Randolph Nesse and evolutionist George Williams, suggest that many allergies that exist today were not common 150 years ago. They say that hay fever was almost nonexistent in England in the early 1800s and rare in Japan even as recently as 1950. Eight types of food account for over 90% of these allergies: milk, eggs, tree nuts, fish, shellfish, soy, and wheat. Thirty percent of children with food allergies also had 1 or 2 other types of allergies-for example, asthma, eczema, or respiratory. A recent study suggests that early exposure to potential allergens may in fact decrease the incidence of allergic responses. Children raised with 2 dogs or 2 cats have fewer allergies than average later in life. An occasional trip to a petting zoo is not sufficient to prevent allergies, but growing up on farm is. Profet, Nesse, and Williams also suggest that if someone is simultaneously exposed, for example, to a plant toxin and an allergen, the immune system may respond to the toxin by producing IgE antibodies. In this attack, the immune system sees the allergen as 'part of the toxin' and so reacts to it as well. Immune cells remain sensitized, and future exposures of the allergen alone will trigger an IgE response even though the toxin is not present. Unfortunately, no matter what the causes or origins of allergies, we must suffer through them with only our allergy medications to help us along. In the United States alone, a transfusion takes place about every 3 seconds-some 12 million units of blood are used every year. Symptoms of a transfusion reaction include fever, low blood pressure, back and chest pain, nausea, and vomiting. Transfusion reactions to other erythrocyte antigens, such as Rh (Rhesus), also occur. However, they usually are less serious than reactions to foreign A or B antigens because the antigen molecules are less numerous. Hemolytic Disease of the Newborn Another example of a cytotoxic reaction is hemolytic disease of the newborn, or erythroblastosis fetalis. Blood with Rh antigens on red blood cells is designated Rh-positive; red blood cells lacking Rh antigens are designated Rh-negative. AntiRh antibodies normally are not present in the serum of either Rh-positive or Rh-negative blood. Sensitization typically occurs when an Rh-negative woman carries an Rh-positive fetus, which inherited this blood type from its father. Careful blood typing and matching of donor and recipient blood prevent most transfusion reactions. Persons with type O blood can safely donate blood to recipients of any blood type. Complement is activated and results in either subsequent phagocytosis or lysis of the red blood cells. Because sensitization usually occurs at delivery, the first Rh-positive child of an Rh-negative mother rarely suffers from hemolytic disease. If this occurs, fetal red blood cells agglutinate, complement is activated, and the red blood cells are destroyed. Such babies exhibit the yellow skin color of jaundice due to excessive bilirubin-a product of the breakdown of red blood cells-in their blood. Hemolytic disease of the newborn can be prevented by giving Rh-negative mothers intramuscular injections of anti-Rh IgG antibodies (Rhogam) within 72 hours after delivery. These anti-Rh antibodies destroy the fetal red blood cells before they can act to sensitize her immune system. It is essential to treat all Rh-negative women after delivery, miscarriage, or abortion in case the fetus may have been Rh-positive. Some people, for example, lack IgA antibodies of their own and thus do not acquire tolerance to this immunoglobulin. When such individuals receive transfusions, they are likely to produce antibodies against IgA in the donated blood, and any subsequent transfusion will then evoke a transfusion reaction. Interestingly some patients exhibit such reactions on receiving their first transfusion, indicating a previous exposure to IgA. One possible explanation is that they have been sensitized by eating rare beef, the blood of which contains IgA. To prevent this situation, Rhogam (anti-Rh antibody) is injected into the mother early in the pregnancy, immediately after delivery, and in cases of miscarriage or abortion. Rhogam reduces exposure to the antigen and thus lessens anti-Rh antibody production. Before anti-Rh antibody (Rhogam) was given preventively, hemolytic disease of the newborn occurred in about 0. Mechanism of Immune Complex Disorders Like anaphylactic and cytotoxic reactions, immune complex disorders also are initiated after sensitization. Antibodies bind to live cells or parts of damaged cells in blood vessel walls and other tissues. Normally, large immune complexes are removed by phagocytosis in the liver and spleen. However, immune complexes are often quite small and fail to bind tightly to Kupffer cells in the liver, thereby escaping elimination from the blood, and are deposited in organs, tissues, or joints. Such antigen-antibody complexes and complement, in turn, cause basophils and mast cells to release histamine and other mediators of allergic reactions, with the effects described earlier. Phagocytes attracted chemotactically to these sites of activity release hydrolytic enzymes, causing tissue damage that is acute but can become chronic if the antigen remains for long periods of time. Under normal circumstances these large immune complexes are engulfed and destroyed by phagocytic cells. Hypersensitivity occurs when antigen-antibody complexes persist or are continuously formed.

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Theoretically treatment without admission is known as purchase 500mg baycip visa, nosocomial infections can be transmitted by all modes of transmission that occur in the community. Exogenous infections are caused by organisms that enter the patient from the environment. They can also be passed on by insects (ants, roaches, flies) from fomites (toilet, trash can) to patients. Other inanimate objects, such as equipment used in respiratory or intravenous therapy, catheters, bathroom fixtures and soap, and water systems, also can be a source of exogenous infections. Some nosocomial infections have even been traced to disinfectants such as quaternary ammonium compounds, to which certain organisms are resistant. Opportunists are most likely to cause infection if the patient has lowered resistance or if normal microflora that compete with pathogens have been eliminated by antibiotics. These organisms are particularly likely to cause such infections because they are ubiquitous (present everywhere) and can survive outside the body for long periods. Universal Precautions apply to the following body fluids: blood, semen, and vaginal, tissue, cerebrospinal, synovial (joint cavity), pleural, peritoneal, pericardial, and amniotic fluids. This is not to imply that no viruses are present in these fluids but rather that the risk of transmission is either very low or unproved. Therefore, some health care facilities require their employees to use Universal Precautions with all body fluids. Infections can arise from a contaminated catheter, inadequate cleaning of the site of catheter insertion, or the movement of organisms from leaky connections. In addition, tubing, joints, containers of fluids, and the fluids themselves also can be contaminated. All surgical procedures expose internal body parts to air, instruments, surgeons, and other operating room personnel, all of which can be contaminated. For example, bacteria that cause pneumonia can reach the lungs from the pharynx during surgery. Respiratory devices, including nebulizer jets, that administer oxygen or air and medications to expand passageways in the lungs provide a means for disseminating microorganisms deep into the lungs. Organisms can grow in the reservoir pans of both cold mist and warm steam humidifiers, and the organisms can be dispersed in an aerosol as the machines operate. Therefore, all respiratory equipment should be disinfected or sterilized daily and, if not disposable, should be disinfected before being moved from one patient to the next. Wear gloves and gowns if soiling of hands, exposed skin, or clothing with blood or body fluids is likely. Wear masks and protective eye wear or chin-length plastic face shields whenever splashing or splattering of blood or body fluids is likely. Discard contaminated needles and other sharp items immediately into a nearby, special puncture-proof container. Clean spills of blood or contaminated fluids by (1) putting on gloves and any other barriers needed, (2) wiping up with disposable towels, (3) washing with soap and water, and (4) disinfecting with a 1:10 solution of household bleach and water. Other devices and procedures account for smaller, but significant, numbers of nosocomial infections. Devices used to monitor blood pressure in the heart or major vessels or cerebrospinal fluid pressure have tubing extending outside the body. Such devices can be contaminated or can allow introduction of organisms from the patient or from the environment. Improperly cleaned gynecological instruments can transmit disease from one patient to another. Endoscopes, which are introduced through body openings and used to examine the linings of organs such as the bladder, large intestine, stomach, and respiratory passageways, are difficult to sterilize and thus can transfer microorganisms from one patient to another. Another important factor that contributes to nosocomial infections is the intensive use of antibiotics, especially in hospital settings. How antibiotics contribute to the development of antibiotic-resistant pathogens and how these pathogens contribute to nosocomial infections were discussed in Chapter 13. Preventing and Controlling Nosocomial Infections the problem of nosocomial infections is widely recognized, and nearly all hospitals now have infection-control programs. In fact, to maintain accreditation by the American Hospital Association, hospitals must have programs that include surveillance of nosocomial infections in both patients and staff, a microbiology laboratory, isolation procedures, accepted procedures for the use of catheters and other instruments, general sanitation procedures, and a nosocomial disease education program for staff members. Safe practice requires using latex gloves, eye protection, a mask to prevent inhalation of aerosols (which are used extensively with modern drills), and the use of sterile equipment. Between patients, all items that could be contaminated with oral secretions or blood must be disinfected. It is not enough for the dentist merely to change gloves between patients-the clean glove might pick up organisms left on the equipment and transport them into the mouth of the next patient. Many dentists now use disposable plastic covers on light adjustment handles and other sites, which can be changed between patients, or wear a head lamp. A University of Georgia study has shown that internal parts of the hollow handle, which holds drill bits, polishing brushes, and other tools, can become coated with blood, saliva, and tooth fragments. The American Dental Association recommends heat sterilization of drill handles between patients, but this is inconvenient, shortens the life span of expensive equipment, and requires having more handles to use with each patient; thus, some dentists do not want to do so. Use of gloves, face shields, and masks by all dental personnel is essential if the spread of infection is to be avoided, but other measures are also necessary. Physicians, nurses, and other staff members who wash their hands thoroughly with soap and water between patient contacts can greatly reduce the risk of spreading diseases among patients. Scrupulous care in obtaining sterile equipment and maintaining its sterility while in use are also important. In addition, the use of gloves when handling infectious materials such as dressings and bedpans and when drawing blood prevents the spread of infections. And, as mentioned earlier, it is important to prevent insect infestations, as flies, ants, or roaches can easily spread an infectious agent. Other techniques are needed to reduce the development of antibiotic-resistant pathogens. Routine use of antimicrobial agents to prevent infections has turned out to be a misguided effort because it contributes to the development of resistant organisms. Antibiotics are given for known infections but are given prophylactically (as preventive measures) only in special situations. Prophylactic antibiotics are justified in surgical procedures such as those involving the intestinal tract and repair of traumatic injuries, in which the surgical field is invariably contaminated with potential pathogens. They also are justified in immunosuppressed and excessively debilitated patients, whose natural defense mechanisms may fail. If all the known techniques for preventing nosocomial infections were practiced rigorously, the incidence of such infections probably could be reduced to half the present level. While most epidemiologists are busy investigating normal ways in which diseases are spread, others are needed to study the ways in which we can protect ourselves from the deliberate spread of disease. In ancient times, infected corpses were flung over the walls of besieged cities, or dumped into wells and other water sources. Gladiators sometimes thrust their swords and tridents into rotting cadavers before entering the arena. This ensured the death of an opponent from even minor cuts, should he kill them first. In more recent times, in 1763 before the American revolution, colonial British military officers gave blankets containing smallpox scabs to native Americans with specific intent to kill them. In April 1979, a Soviet anthrax factory in the Ural Mountains, in a city then called Sverdlovsk but now returned to its earlier name of Yekaterinburg, had a tragic accidental leak that caused an anthrax epidemic. Complete details of this event were found in Anthrax, the Investigation of a Deadly Outbreak, by Jeanne Guillemin, 1999, University of California Press. In 1984, in Oregon, members of a religious cult known as the Rajneeshees caused outbreaks of Salmonella typhimurium food poisoning by sprinkling organisms on the salad bars of ten local restaurants, a nursing home, and even the local medical center. They even gave the investigating judge a glass of water containing Salmonella, in hopes of killing him. More details of this and other attacks can be found in Germs, by Judith Miller et al. Ricin is not microbial, but rather is an extract from the castor bean, Ricinus communis. Preparations for homeland defense against bioterrorism have created more questions than answers. During January 24 to December 31, 2003, smallpox vaccine was given to 39,213 civilian health workers.

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Prevention by eradicating mosquito vectors is a more appropriate means of decreasing the already low incidence of encephalitis among humans symptoms 2dpo baycip 500 mg without a prescription. Herpes simplex virus, which usually is responsible for cold sores, also can cause herpes meningoencephalitis. This disease often follows a generalized herpes infection in a newborn infant, child, or adult. The disease has a rapid onset with fever and chills, headache, convulsions, and altered reflexes. In the middle-aged or elderly, meningoencephalitis causes confusion, loss of speech, hallucinations, and sometimes seizures. The viremia that follows allows the viruses to reach target organs, particularly the kidneys, lungs, and brain. Polyomaviruses, which are papovaviruses, were first recognized in the 1960s as viral particles in the enlarged nuclei of oligodendrocytes. These are the cells that produce myelin, the lipoprotein that coats nerve fibers in the central nervous system. Infected oligodendrocytes are observed to surround areas that lack myelin in the brains of patients dying from progressive multifocal leukoencephalopathy. Diagnosis is difficult because cerebrospinal fluid remains normal, and only nonspecific changes are seen in electroencephalograms. On occasion a young patient develops this disease as a complication of a primary infection, but most cases result from reactivated latent viruses from childhood infections. However, unchecked viral multiplication, which is particularly likely with T cell deficiencies, can sometimes occur and cause clinically apparent disease. No diagnostic tests for polyomaviruses are available for routine use; nor is any treatment available for the infections, even if they can be recognized. The amoebas probably enter through the nasal passage and make their way up along nerves to the meninges. Acanthamoeba polyphaga accumulates on the water surface of the contaminated hot tubs when the tubs are covered. These amoebas cause ulceration of the eyes and the skin; if they invade the central nervous system, death can occur within a few weeks due to meningoencephalitis. Many so-called leprosy cases were other skin diseases such as fungal and viral infections, and the houses probably had fungi growing on their walls. Five countries account for 90% of the cases: Brazil, Madagascar, Mozambique, Tanzania, and Nepal. No reliable test is available to disclose all these subclinical cases, although the lepromin skin test, similar to the tuberculin skin test for tuberculosis, detects some of them. A pin may be stuck into these "anesthetized" areas and not be felt because of destruction of nerves and nerve endings. Incubation time averages 2 to 5 years for the tuberculoid form and 9 to 12 years for the lepromatous form. Mycobacterium leprae is the only bacterium known to destroy peripheral tissue; it also destroys skin and mucous membranes. The organism has a predilection for cooler parts of the human body, such as the nose, ears, and fingers, but large numbers of organisms are seen throughout the body, except for the central nervous system. Continuous bacteremia of 1,000 organisms per milliliter of blood has been demonstrated in lepromatous cases. Large numbers of bacilli are shed in respiratory secretions and in pus discharged from lesions. This deformity can be treated surgically in its early stages, thereby preventing crippling. Public Health Service facility in Carville, Louisiana, has pioneered development of these special surgical techniques. In the past decade, it was recognized that changes in the feet of diabetic patients can be helped by these same surgical techniques. Thus, Carville now has an active teaching program for surgeons who will use knowledge gained from one of the most shunned diseases to help thousands of victims of diabetes. It is clear that the disease traveled from the Old World to the New World and that in the past, even allowing for misdiagnosis, its incidence in Europe was much higher than it is today. In England, lepers were shunned even in death, being buried in separate cemeteries. The disease is treated with dapsone, clofazimine, and rifampin, but dapsone-resistant strains are beginning to appear. Treatment greatly reduces nodules of lepromatous disease, but it cannot restore lost tissue. Now the disease can be arrested, and the people can live nearly normal lives without infecting others in their community. Strong responses and distinctly positive skin tests are seen in patients with the less serious tuberculoid disease. Weak responses and negative skin tests are seen in patients with rapidly progressing lepromatous disease. However, test results may change over time from positive to negative and vice versa as immune response rises and falls. Lepromatous patients usually have adequate cell-mediated response to other antigens, so their lack of immunity is not due to generalized T cell absence or dysfunction. The absence of T cells in "nude" mice, which are hairless and lack a thymus, makes them suitable organisms for growing large quantities of organisms. Once reprogrammed, the Schwann cells lose their ability to protect neurons, leaving them unable to transmit impulses, thus causing degeneration of the nervous system. Furthermore, the reprogrammed cells are able to turn into other cell types, such as bone and muscle, thus aiding in the spread of M. They secrete chemokines that attract immune cells, which engulf them and spread them even farther. Avoiding exposure and receiving prophylactic chemotherapy after exposure are the only means of prevention. However, a group called the Global Alliance for Leprosy Elimination expects to treat and cure all leprosy patients remaining in the world by distributing free drugs for this purpose! The organism can be cultured in the laboratory only under strict anaerobic conditions. Clostridium tetani endospores are exceedingly resistant to drying, disinfectants, and heat. Therefore, handling bedpans, dirty diapers, or other objects contaminated with feces can transmit organisms to persons who have any breaks in their skin. Since the development of tetanus vaccine in 1933, the incidence of tetanus in the United States has steadily dropped, with the annual number of cases remaining below 100 since 1975, and only 8 in 2010. Vaccine was not available during their childhoods, and they did not receive it during military service, as men did. They remain susceptible to tetanus spores as they enjoy gardening in their retirement years. To cause tetanus, spores must be deposited deep in tissues, where oxygen is unavailable. Stepping on a rusty nail has a reputation for leading to tetanus, but it is tetanus endospores, not rust, that cause the disease- a shiny new nail can be just as dangerous if the spores are present. Making puncture wounds bleed helps flush tetanus spores and other organisms from them. Once inside the host, the noninvasive tetanus organisms stay at the wound site and release a powerful exotoxin; tetanus is a toxin-mediated disease. Eventually, respiratory muscles become paralyzed, heart function is disturbed, and, with rare exceptions, the patient dies. On battlefields strewn with horses and manure, contamination of wounds with tetanus spores was inevitable. Antitoxin and antibiotics are given to nonimmunized patients when injuries are treated. Because antitoxin must be administered to inactivate the toxin before the immune system has time to become sensitized to it, infection treated in this way confers no immunity. In some societies, contaminated knives are used to cut umbilical cords after a baby is delivered, and mud is smeared on the cut end. In parts of some developing nations, 10% of deaths within a month of birth are due to neonatal tetanus.

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Discussions of destroying the 'last two' remaining stocks of smallpox virus have since fallen by the wayside 2c19 medications order baycip in india. During a 12-day incubation period, it infects phagocytic cells and later blood cells. They then rapidly spread to the face, forearms, hands, and finally the trunk and legs. As the disease progresses, the blisterlike lesions become opaque and eventually form crusts that fall off. In a nonimmune population, smallpox is highly contagious; in a largely immunized population, it spreads very slowly. But now that smallpox has been eradicated worldwide, complications of vaccination are far more life-threatening than the disease itself. Cowpox, caused by the vaccinia virus, causes lesions (similar to a smallpox vaccination) at abrasion sites, inflammation of lymph nodes, and fever. Vaccinia viruses can also cause a progressive disease, with numerous lesions and symptoms more like those of smallpox, that can be treated with the drug methisazone. Not only was the vaccinia virus used by Jenner to immunize against smallpox, but also it was the first animal virus to be obtained in sufficient quantity for chemical and physical analysis (Chapter 1, p. Monkeypox is sometimes mistaken for smallpox, as the lesions and death rates are very similar. Usually it occurs in western and central Africa, especially in Zaire and the Congo. Smallpox and monkeypox are both orthopoxviruses, so it is not surprising that smallpox vaccination protects against both. With discontinuation of smallpox vaccination, we are now seeing outbreaks of monkeypox among zoo monkeys (which is how it got its name), and most recently among Americans who purchased African giant rats at pet shops. These outbreaks are an unlooked-for consequence of cessation of smallpox vaccination, and can be a serious threat in themselves. Some people suggest that smallpox immunization be resumed in areas where monkeypox is endemic. First, it differs immunologically from both orthopoxviruses and parapoxviruses, the two major groups of poxviruses. Thus, this virus may be intermediate between viruses that cause specific diseases and those that induce tumors. The disease usually affects children and young adults, and it can persist for years. It is acquired by personal contact or from items such as gym equipment and swimming pools. Treatment generally involves the removal of growths by chemicals or by localized freezing. Warts grow freely in many sites in the body-the skin, the genital and respiratory tracts, and the oral cavity. Even when warts disappear or are removed, the virus remains in surrounding tissue, and warts may recur or form malignant tumors. Some are barely visible and self-limiting-that is, they do not grow or spread- and others, such as laryngeal warts, are larger but benign. But other strains of the same virus, which do not produce visible warts (remaining as chronic "invisible" infections), do cause 99% of all cases of cervical cancer. Papillomaviruses are transmitted by direct contact, usually between humans, or by fomites. Genital warts are sexually transmitted, and juvenile onset larynge al warts are acquired during passage through an infected birth canal. According to Lewis Thomas, warts 'can be ordered off the skin by hypnotic suggestion. Within a few weeks, this happened to 9 patients, including someone who 'got mixed up and destroyed the warts on the wrong side. Only a few warts are present at any one time, and most regress in less than 2 years. Warts can be distinguished by immunological tests and microscopic examination of tissues. Enzyme immunoassay and immunofluorescent antibody tests can detect about three-fourths of the cases in which viruses are found microscopically. These tests sometimes fail because certain papillomas, especially genital and laryngeal papillomas and those progressing toward malignancy, produce only small quantities of antigens. The most widely used treatment, cryotherapy, involves the freezing of tissue with liquid carbon dioxide or liquid nitrogen and excision of the infected tissue. Caustic chemical agents such as podophylin, salicylic acid, and glutaraldehyde; surgery; antimetabolites such as 5-fluorouracil; and interferon to block viruses also are used to get rid of warts. It is caused by coxsackievirus A16, although a more severe form is caused by coxsackievirus A6. The symptoms are rashes on the hands and feet, blisterlike sores in the mouth, and fever. Following a severe case, the skin may peel, and the fingernails can fall off-but they will grow back. Since 1964, only 129 cases of protothecosis had been reported, 2 from cleaning home aquariums. Protothecosis was first observed on the foot of a rice farmer, and most subsequent cases have occurred on legs or hands. In immunodeficient patients, the parasite can invade the digestive tract or peritoneal cavity. A few skin infections have responded to oral potassium iodide or intravenous amphotericin B and tetracycline therapy, but no satisfactory treatment has been found for others. Fungal Skin Diseases the fungi that invade keratinized tissue are called dermatophytes (durmato-fitz), and fungal skin diseases are called dermatomycoses (durma-to-mi-koses). These diseases can be caused by any one of several organisms, mainly from three genera: Epidermophyton, Microsporum, and Trichophyton Table 19. Fungi that invade subcutaneous tissues live freely in soil or on decaying vegetation and can be found in bird droppings and as airborne spores. Subcutaneous fungal infections usually spread slowly and insidiously; response to treatment is likewise slow. None of the dermatomycoses result in severe diseases, and lesions usually do not invade other tissues, but they are unsightly, itchy, and persistent. For example, ringworm of the scalp is easily acquired at hairstyling establishments if strict sanitary practices are not followed. Ringworm involves the skin, hair, and nails, and most forms are named according to where they are found. Tinea unguium (ringworm of the nails) causes hardening and discoloration of fingernails and toenails. In tinea capitis (scalp ringworm), hyphae grow down into hair follicles and often leave circular patterns of baldness. Tissue damage caused by dermatophytes can allow secondary bacterial infections to develop. Diagnosis of ringworm can be made by microscopic examination of scrapings from lesions, but observation of the skin itself is often sufficient. Although fungi in tissues generally do not form spores, those in laboratory cultures often do, and workers must be especially careful not to become infected from escaping spores. Treatment generally consists of removing all dead epithelial tissues and applying a topical antifungal ointment, such as Lotrimin powder or Tinactin. If lesions are widespread or difficult to treat topically, as when they infect nailbeds, griseofulvin is administrated orally. Subsequently, the skin cracks and peels, and a secondary bacterial infection leads to itchy, soggy, white areas between the toes. But in dandruff sufferers, the cells flake off in big clusters in only 7 to 10 days, never fully maturing. The proper way to use these is to apply them to a dry scalp, massage them in thoroughly, let sit for 10 minutes, and only then add water.

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Other monoclonal antibodies allow rapid diagnosis of hepatitis medicine klonopin cheap baycip 500mg overnight delivery, influenza, and herpesvirus and chlamydial infections. Diagnostic tests for other infectious diseases and allergies are being developed at a rapid rate, and progress is being made in using monoclonal antibodies to diagnose various kinds of cancer. Some of the cancers for which monoclonal antibodies are currently used to monitor treatment or for diagnosis include prostate cancer, colorectal cancer, testicular cancer, thyroid cancer, lymphomas, myelomas, and small cell lung cancer. These methods first require preparing antibodies to infectious agents or malignant cells. Then an appropriate drug or radioactive substance must be attached to the antibodies. If such antibodies are given to a patient, they carry the toxic substance directly to the cells that bear the appropriate antigen. The great advantage of therapeutic monoclonal antibodies is that they selectively damage infected or malignant cells without damaging normal cells. Monoclonal antibodies are also being used to prevent respiratory syncytial virus infections in children, prevent acute kidney transplant rejection, and to treat rheumatoid arthritis. Monoclonal antibodies against tumor antigens have been tried in a few cancer patients. Unfortunately, the patients often displayed allergic reactions to myeloma proteins that accompany the antibodies. Researchers are now producing "humanized" monoclonal antibodies, which will kill malignant cells without causing allergic reactions in the patients that receive them. Humanized monoclonal antibodies, put together by genetic engineers, have a human constant region, plus a variable region made up of human and mouse portions. Diphtheria exotoxin delivered to cancer cells by monoclonal antibodies is being tried as a therapy for cancer. In cell-mediated immunity, T cells interact directly with other cells that display foreign antigens. These interactions clear the body of viruses and other pathogens that have invaded host cells. They also account for rejection of tumor cells, some allergic reactions, and immunological responses to transplanted tissues. The cell-mediated immune response involves the differentiation and actions of different types of T cells and the production of chemical mediators called cytokines (lymphokines, interleukins). Much recent research has been devoted to determining the characteristics, origins, and functions of T cells, including the functions of secreted cytokines. However, they do have a particular cell membrane receptor protein that corresponds to the antibodies of B cells and other receptor proteins as well. The Cell-Mediated Immune reaction Cell-mediated immunity involves the response of T lymphocytes. The cell-mediated immune reaction typically begins with the processing of an antigen-usually one associated with a pathogenic organism-by dendritic cells, B cells, or macrophages. When macrophages and dendritic cells phagocytize pathogens, they ingest and degrade the pathogen. Pieces of the pathogen, peptides, are then transported to the surface of the macrophage or dendritic cell. When a macrophage presents the antigen to T cells that have the proper antigen receptor, the antigen and receptor bind. Each cell is sensitized to the antigen that initiated the process, and each type has a different function in cell-mediated immune reactions. Some cells act directly and others release leukotrienes or cytokines, which are chemical substances that trigger certain immunologic reactions. Refer to the figure as you read about the functions of different kinds of T cells. At the same time that these cells are differentiating, some T memory cells also are being formed. As in humoral immunity, the persistence of memory cells in cell-mediated immunity allows the body to recognize antigens to which T cells have previously reacted and to mount more rapid subsequent responses. Other regulatory cells, and the disappearance of foreign antigen as the immune response proceeds, apparently help to prevent both humoral and cell-mediated immune processes from getting out of hand. Migration inhibiting factor, which prevents macrophages from leaving sites of infection. Macrophage aggregation factor, which causes macrophages to congregate at such sites. When pathogens have evaded humoral immunity and established themselves inside cells, they can cause long-term infections unless the infected cells are destroyed by cellmediated immunity. An agent that infects T cells is especially devastating because it destroys the very cells that might have combated the infection. Eosinophils have a similar protein, which they may use to kill certain helminths and other parasites. Cytotoxic T cells bind to antigens presented by macrophages and then attack virus-infected cells. Both kinds of killer cells contain granules of a lethal protein, perforin, which is released when they bind to a target cell. Perforin bores holes in the target cell membranes so that essential molecules leak out and the cells die. By killing infected cells while they are few in number and before new virus particles are released from them, cytotoxic T cells prevent the spread of infection-but at the expense of destroying host cells. Both kinds of killer cells can withdraw from cells they have damaged and move on to other target cells. This factor causes macrophages to increase production of toxic hydrogen peroxide, along with enzymes that attack the phagocytized organisms and accelerate the inflammatory response. We have now completed the discussion of cellmediated immunity-how it is initiated and how its effects are produced. The discovery of such an efficient mechanism for killing cells raises two important questions: What prevents perforin from killing adjacent uninfected cells, and what prevents it from attacking the membranes of the killer cells themselves Binding to the receptor molecule does not involve specificity for the receptor site. The greatest number of T cells that are activated in this way do not have a use in fighting the causative infection. Many of them die as a result, leaving the immune system deficient in those types of cells, thus leaving the host open to even more infections. When these multiply excessively, the host tissues are attacked-a disorder called autoimmunity, which will be discussed in Chapter 18. It is possible that autoimmune diseases such as rheumatoid arthritis and multiple sclerosis may be caused by superantigens. It consists of the entire gastrointestinal tract, urogenital tract, respiratory tract, and mammary glands. The immune response to pathogens on the epithelial surface of the mucosa has some characteristics different than the immune response to pathogens in the blood and lymph. They take up antigens from the gut by endocytosis and release the antigens to antigen-presenting cells, such as dendritic cells beneath them. Previously unactivated lymphocytes that become activated by the antigen-presenting cells are transported to other mucosal surfaces by entering the blood, via lymph nodes that drain from the intestinal region (mesenteric lymph nodes) or thoracic region. The primary immunoglobulin on mucosal surfaces, breast milk, and colostrum is IgA. Interestingly, the uterus, a part of the urogenital tract, is a privileged site- that is, it is isolated from the adaptive immune system. Factors That Modify Immune responses the host defenses of young, healthy, human adults living in an unpolluted environment are capable of preventing nearly all infectious diseases. However, a variety of disorders, injuries, medical treatments, environmental factors, and even age can affect resistance to infectious diseases. In the beginning of this chapter we noted that humans are genetically immune to some diseases. It has also been found that different races have different degrees of resistance and susceptibility to various diseases. In general, the very young and the elderly are most susceptible to infections, and young adults are least susceptible. The young are susceptible because the immune system is not fully developed until age 2 or 3.