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18 Cards in this Set

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Explanation (Hint):
The World Health Organization identifies an alarming increase in hospital admissions worldwide attributable to a new and unexpected serotype of influenza A virus. The biological attribute of influenza A virus, which allows the sudden appearance of dramatically new genetic variants, is also present in a limited number of other viral families. Which of the following viruses also possesses this biological attribute?
*This case describes a pandemic of influenza A, which is caused by the ability of the virus to undergo dramatic genetic changes of type by reassortment of its segmented RNA genome--a trait called genetic shift. The only virus on the list that possesses a segmented genome is the rotavirus, in the reovirus familiy, which possesses 10-11 segments in its genome. *My answer: coronavirus (incorrect); Coronavirus is NOT segmented and is a cause of the common cold. *HIV is NOT segmented and is known for its genetic drift (minor mutational changes over time due to an error-prone polymerase), NOT genetic shift. *Measles virus is NOT segmented and is controlled largely by vaccination. *Rabies virus is NOT segmented *Rubella virus is NOT segmented *St. Louis encephalitis virus is a flavivirus and is NOT segmented.
A 37-year-old newly married man presents with multiple blister-like lesions on the glans of his penis, appearing over the past 2 days. On questioning, he recalls similar episodes over the past 2 years. Examination is remarkable for tender, 3-4 mm vesicular lesions on the shaft of his penis with no apparent crusting, drainage, or bleeding. There is also slight bilateral inguinal adenopathy. During the asymptomatic period between outbreaks, where would the causative agent likely have been found?
Neurons of the sacral ganglia
This is a classic example of an infection with herpes simplex virus (probably type 2). This agent causes lytic infections in mucoepithelial cells. It is then retrogradely transported into neurons of the sacral ganglia, where it lays dormant during the asymptomatic phase of the disease. *Fibroblasts may be infected by CMV, another herpesvirus, but this produces a distinctive mononucleosis-like syndrome in most normal individuals. *My answer: Lymphocytes and macrophages may be infected by herpes simplex type 2 but are not the site of residence of the virus during quiescent periods. *Infection of mucoepithelial cells by herpes simplex produces vesicular-like lesions on the mucous membranes during symptomatic periods. *Neurons of the trigeminal ganglia may be latently infected with herpes simplex type 1. This agent is a possible cause of genital ulcers, but is usually associated with perioral lesions (cold sores).
A veterinary student at a state university is referred to the student health clinic with complaints of fatigue, malaise, and lymphadenopathy. She has a fever of 38.2 C (100.8F), pronounced cervical lymphadenopathy, and a moderate lymphocytosis. She is three months pregnant with her first child. She is concerned with the potential for having become infected with Toxoplasma gondii. Which stage of the parasite is capable of crossing the placenta?
Toxoplasma gondii is capable of crossing the placenta and causing harmful fetal infection only during the acute maternal infection. During this period, rapidly dividing forms called tachyzoites can be spread via the bloodstream and thus access the placenta. As soon as the maternal immune response is initiated, parasitemia ceases, and the parasite can no longer enter the fetus. *Bradyzoites are slowly-dividing forms of the parasite that develop in the tissues of individuals who have a protective immune response. They are extremely long-lived, and remain in an almost dormant fashion in the tissues of a person recovering from an acute infection with Toxoplasma. They are not bloodborne and cannot cross the placenta. *Merozoite is a general term for a daughter parasite within the sporozoan group of parasites. Tachyzoites and bradyzoites are both types of merozoites, but merozoite is not the best, most precise answer here. *Sporozoites are the infectious forms of the sporozoan parasites. In the case of Toxoplasma, these are passed in the feces of cats, inside oocysts, and once accidentally ingested, they can produce infection in the tissues of man and other animals. Once in the tissues, they multiply rapidly into intracellular cysts forms filled with tachyzoites. *Trophozoite is the term used to describe the vegetative, feeding form of many types of protozoan parasites. It is distinguished from the infectious form in that it actively feeds, grows, and divides inside the host. Although tachyzoites, bradyzoites, and merozoites are sub-categories of trophozoites, this is not the most specific term fo rthe form that crosses the placenta, and thus, NOT the best answer here.
A 54-year-old woman suffering from influenza deteriorates and develops shaking chills and a high fever. Physical examination is remarkable for dullness to percussion at the left base and decreased breath sounds on the left. Chest x-ray confirms the diagnosis of lobar pneumonia, presumed to be caused by Streptococcus pneumoniae. The patient has no known drug allergies. Which of the following antibiotics would be most appropriate to treat the patient's condition?
Penicillin remains the first-line drug of choice for pneumococcal pneumonia, EXCEPT in patients with penicillin allergy and in the relatively few areas in which pneumococcal strains with high-level penicillin resistance exist. Alternative therapies include erythromycin and vancomycin. *My answer: The third-generation cephalosporin CEFOTAXIME is NOT usually used for pneumococcal pneumonia. *Chloramphenicol is NOT usually used for pneuococcal pneumonia *Erythromycin is a good alternative therapy for pneumococcal pneumonia, but is usually used only when a penicillin allergy is present. *Vancomycin is NOT the first-line therapy, but it is a good alternative in patients allergic to penicillin or when high-level penicillin resistance (relatively uncommon) is present.
A patient with rheumatoid arthritis presents to her physician and mentions that after many years without teeth problems, she has recently developed seven caries. This is a clue to her clinician that she should be evaluated for which of the following diseases?
Sjogren's syndrome
Rheumatoid arthritis can coexist with a variety of autoimmune diseases (including those listed in the answers), but is most frequently associated with Sjogren's syndrome. Sjogren's syndrome is due to autoimmune involvement with subsequent scarring of the salivary and lacrimal glands, leading to dry eyes and dry mouth. Secondary effects include parotid gland enlargement, dental caries, and recurrent tracheobronchitis. *Squamous cell carcinoma of the mouth is NOT associated with dryness of the mouth. *Polyarteritis nodosa is a systemic necrotizing vasculitis. Patients present with low-grade fever, weakness, and weight loss. They may also have abdominal pain, hematuria, renal failure, hypertension, and leukocytosis. *My answer: SLE is an autoimmune disease characterized by vasculitis (which may produce a variety of symptoms depending on the site of the lesion), rash, renal disease, hemolytic anemia, and neurologic disturbances. *Thyrotoxicosis produces insomnia, weight loss, tremors, heat intolerance, excessie sweating, and frequent bowel movements or diarrhea.
A 46-year-old woman complains to her physician at the time of her annual examination of fatigue and painful lymph nodes in her neck, which she says have been present for at least 6 months. A lymph node biopsy reveals hypercellularity in the cortical areas, and serum electrophoresis shows a spike of protein in the gamma region. The abnormal immunoglobulin is determined to be of the IgG2 isotype. An IgG2 molecule is composed of which of the following?
Two gamma2 chains and two kappa chains
IgG molecules contain two gamma heavy chains of a given subtype and two light chains (either kappa or lambda). The 2 in IgG2 indicates the subclass to which the molecule belongs. IgG2 contains two gamma2 chains (since a given B cell can only form one type of heavy chain). The IgG molecule will contain either two kappa chains or two lambda chains, but never one of each. *My answer: One alpha, one gamma2, and two kappa chains is NOT correct because a given cell produces immunoglobulin molecules with a single type of heavy chain. *One gamma1 chain and two kappa chains is NOT correct. Each immunoglobulin has two heavy chains and two light chains, so this option lacks a heavy chain to make a complete IgG1 molecule. *Two gamma1 chains and one kappa and one lambda chain is NOT correct, since an immunoglobulin molecule must contain two identical heavy chains and two identical light chains. *Two gamma1 chains and two kappa chains is NOT correct, since this describes a complete IgG1 molecule.
A 67-year-old black man with a history of glucose-6-phosphate dehydrogenase (G-6-PD) deficiency presents with fever, irritative voiding symptoms, and perineal pain. Rectal examination is remarkable for a boggy, exquisitely tender prostate. A urine Gram's stain is positive for gram-negative rods. The risk for development of hemolytic anemia is highest if he receives high-dose, 21-day therapy with which of the following?
The patient has acute prostatitis, which is characterized by fever, chills, and dysuria, with a swollen, extremely tender prostate on rectal exam. The urine Gram's stain and culture will generally be positive. The treatment regimen for this bacterial infection is typically a 21-day course of ampicillin, a fluroquinolone, or sulfamethoxazole/trimethoprim (SMX-TMP). G-6-PD deficiency is an X-linked recessive disorder affecting 10% to 15% of American black males. The medications most commonly associated with the induction of hemolytic anemia in deficient patients are sulfonamides, nitrofurantoin, dapsone, primaquine, and quinine. The sulfamethoxazole in the SMX-TMP combination is sulfonamide, and can produce hemolytic anemia in patients with G-^-PD deficiency. *Ampicillin is a broad-spectrum penicillin antibiotic commonly used in the treatment of infections in the genitourinary, respiratory, or GI tracts, as well as in the skin and soft tissues. *Cefaclor is a second-generation cephalosporin indicated for a variety of bacterial infections, including those of the respiratory and GI tracts. *Ciprofloxacin is a fluroquinolone commonly used in the treatment of serious infections caused by gram-negative organisms. *MY ANSWER: Tetracycline is most commonly used in the treatment of acne vulgaris and gonoccoal infections.
A previously healthy 27-year-old man presents with one week of increasingly severe sore throat, fever, fatigue, and headache. His temperature is 39 C (102.2F) orally. He has palpable anterior cervical adenopathy and petechiae over his trunk and oral mucosae but lacks a pharyngeal exudate. His liver is palpable 3 cm below the right costal margin and his spleen tip is also palpable. His sclerae are anicteric. A blood smear appears as shown above. What is the identity of the predominating white blood cell?
CD8+ lymphocyte
This is a case of infectious mononucleosis caused by the Epstein-Barr virus. This virus infects B lymphocytes by attaching to the CD21 receptor, but the cells that will predominate in a blood film will be reactive T cells, which would be CD8+ T lymphocytes. *Basophil would be a cell that would be increased in cases of allergic response. *CD16+ lymphocytes would be NK cells, which are NOT inducible, and therefore would NOT be increased in number, regardless of infection. *CD21+ lymphocytes are the cells infected and destroyed by the virus. *Eosinophils are cells increased in allergic or parasitic responses. *MY ANSWER: Monocytes are the cells infected by CMV, the other cause of mononucleosis, which is less likely to be symptomatic in healthy adults. *Polymorphonuclear leukocytes are the cells most likely to be increased in bacterial infections.
A 24-year-old man presents with complaints of itching on his arms and face. Physical examination reveals well-circumscribed wheals with raised, erythematous borders and blanched centers. Which form of hypersensitivity is this patient probably exhibiting?
Immediate type hypersensitivity
Urticaria (hives) is a good example of a local anaphylaxis reaction, which is classified as a Type I hypersensitivity reaction. Type I hypersensitiviy reactions involve preformed IgE antibody bound to mast cells or basophils, which release vasoactive and spasmogenic substances when they react with antigens. *Acute serum sickness is now uncommon but was formerly seen when animal sera were used for passive immunization. *The eosinophil-mediated cytotoxicity against parasites is an example of antibody-dependent cell-mediated cytotoxicity. *Myasthenia gravis is an example of a disease caused by anti-receptor antibodies *MY ANSWER: The tuberculin (PPD) REACTION USED TO TEST FOR TUBERCULOSIS EXPOSURE IS AN EXAMPLE OF DELAYED-TYPE HYPERSENSITIVITY.
A 6-year-old child is brought to the pediatrician's office complaining of a severe sore throat. ON examination, the child is febrile, and the throat is extremely erythematous with obvious abscesses on the tonsillar pillars. A rapid antigen test is positive for Streptococcus pyogenes. At which stage of the above growth curve would a beta-lactam antibiotic have greatest efficacy?
log phase
Beta-lactam drugs act by inhibiting cell wall synthesis. In a bacterial culture, cell wall synthesis will occur at a maximal rate during the period of exponential growth, which is shown on the graph by letter C. *MY ANSWER: The lag phase is the initial phase of a bacterial culture during which there is no increase in cell number. Cells in a culture at this stage are activating enzymes necessary for metabolism, but are not undergoing cell wall synthesis, and would NOT be susceptible to beta-lactams. *The acceleration phase is the initial phase of bacterial multiplication, when cell division has not reached a maximum. Althgouh cell wall synthesis would indeed make such cells sensitive to beta-lactam antibiotics, it is not the period of maximum sensitivity. *The retardation phase is the phase of the bacterial growth curve when toxic metabolic products begin to accumulate and nutrient levels begin to be insuffienct to support continued growth. New cell wall synthesis would be beging to fall at this point, and so sensitivity to beta-lactams would be less than that during the phase of exponential growth. *The maximum stationary phase is a period of the bacterial grwoth curve when the number of bacteria dividing in the culture equals the number of cells dying. This would not be a period of intense cell wall synthesis, and thus cells in such a culture would not be extremely sensitive to beta-lactams. *The decline and death phases of the bacterial growth curve are periods when bacteria are dying more rapidly than they are dividing. Cell wall synthesis would be minimal, and cultures would not be sensitive to beta-lactams.
In a random quality-control test of 73 batches of Hemophilus influenzae vaccines administered nationwide, the FDA discovered two lots in which the capsular polysaccharide of Hemophilus was not covalently coupled to the Diptheria toxoid. Which of the following immunologic responses would be predicted in children receiving one of these unusual vaccine lots?
They should produce a normal IgM response to Hemophilus
The function of covalently coupling the capsular polysaccharide of Hemophilus to the Diptheria toxoid in this common childhood vaccine is to involve T lymphocytes to assist in the production of antibodies against Hemophilus. T lymphocytes respond to processed peptides presented in the groove of major histocompatibility antigens on antigen-presenting cells, and are necessary for causing class switching in B lymphocytes. Therefore, in the absence of T cell stimulation, B lymphocytes will make only IgM, and will be incapable of switching to other isotypes of immunoglobulin production. *A normal DTH response is incorrect because DTH is a T-cell mediated immune response, and without covalent coupling to a protein material, T cells will NOT recognize a polysaccharide antigen. *A normal IgA response is incorrect because the production of IgA is dependent on the stimulation of T lymphocytes, which is not possible without covalent coupling between the protein and the polysaccharide. *A normal IgG response is incorrect because the production of IgG is dependent on the stimulation of T lymphocytes, which is not possible without covalent coupling between the protein and the polysaccharide. *A normal NK response to Hemophilus is NOT the best answer because NK cells are elicited in reponse to some tumors and viruses, but NOT typically to bacterial antigens. *MY ANSWER: No response to Hemophilus is NOT true, because even a person without T cells will make an IgM response to a polysaccharide antigen. It will NOT be a large response, but it will be present, simply due to the random recognition of this antigen by naive B lymphocytes being produced by the bone marrow.
A 48-year-old waitress presents to a physician with malaise, loss of appetite, nausea, moderate fever, and jaundice. Laboratory tests indicate a marked increase in serum transaminases. Serology for hepatitis viruses is performed and indicates positive results for the presnce of HBsAg, HBc IgM antibody, and HCV antibody. Antibody tests for HBsAb and HAV are negative. The results indicate:
The presence of an acute HBV infection
The presence of hepatitis B surface antigen (HBsAg) along with hepatitis B core IgM antibody (HBc IgM Ab), and the absence of hepatitis B surface antibody (HBsAb) indicates the presence of the early stages of an acute infection with Hepatitis B. The presence of antibody to HCV only indicates exposure, but not a specific time of exposure; however, 85% of patients who are infected with HCV develop chronic infections, indicating that this patient has an 85% chance of having a dual infection with HBV and HCV. The acute or chronic HCV infection can be confirmed by PCR. *A dual infection of HBV and HAV is NOT plausible since the IgM anti-HAV serology is negative. *Hepatitis A does NOT cause chronic disease. *MY ANSWER: Chronic HBV infection is unlikely because the patient has HBC IgM Ab, which is characeristic of an acute infection, rather than a chronic infection. *Hepatitis C infection is NOT confirmed by these data because the presence of HCV Ab only inidicates exposure to the virus, and not the state of infection. This could be caused by exposure at some earlier time (the elevated serum transaminases might be due to HBV infection). An active or chronic HCV infection can only be confirmed by PCR.
A febrile 23-year-old college coed presents with fatigue and difficulty swallowing. Physical exam reveals exudative tonsilitis, palatal petechiae, cervical lymphadenopathy, and tender hepatosplemonegaly. A complete blood count reveals mild anemia, lymphocytosis with about 30% of the lymphocytes exhibiting atypical features, and a mild thrombocytopenia. Coombs' test is positive. Which of the following is the most likely complication of this syndrome?
splenic rupture
The syndrome represented by the clinical vignette is infectious mononucleosis. Epstein-Barr virus is the usual cause of heterophile-positive infectious mononucleosis; cytomegalovirus is responsible fo ra minority of cases. Rarely, splenic rupture requiring splenectomy can result form splenomegaly and capsular swelling, usually occurring during the 2nd and 3rd weeks of the illness. *MY ANSWER: Acute cholecystitis is NOT associated with infectious mononucleosis. The most frequently isolated pathogens are E. coli, Klebsiella spp., group D Streptococcus, Staphylococcus spp., and Clostridium spp. *Ascending cholangitis is NOT associated with infectious mononucleosis. Cholangitis usually presents with biliary colic, jaundice, and spiking fever with chills (Charcot's triad). Blood cultures are usually positive (E. coli is a common isolate), with an accompanying leukocytosis. AIDS-related cholangitis has been reported, presenting with abdominal pain and obstructive liver symptoms. Potential etiologic agents include Cytomegalovirus, Cyrptosporidium parvu, and Microsporidia, including Enterocytozoon cuniculi. *Diarrhea is NOT usually produced by infectious mononucleosis. *Immune complex vasculitis is NOT associated with infectious mononucleosis.
A 25-year-old pregnant woman in her third trimester is diagnosed with an upper respiratory infection. Which of the following would be the most appropriate pharmacotherapy?
The Food and Drug Administration (FDA) has established a set of pregnancy categories to determine the rational use of any medication to be administered during pregnancy that requires a risk versus benefit assessment. Regardless of the designated "Pregnancy Category" or presumed safety, no medication/drug should be administered during pregnqancy unless it is clearly needed and the potential benefits outweight the potential hazards to the fetus. The following table summarizes the established pregnancy categories. *Pregnancy Category A: Adequate studies in pregnant women have not demonstrated a risk to the fetus in the first trimester of pregnancy, and there is no evidence of risk in later trimesters. *Pregnancy Category B: Animal studies have NOT demonstrated that a risk to the fetus exists, although there are no adequate studies in pregnant women. Or, animal studies have shown an adverse effect, but adequate studies in pregnant women have not demonstrated a risk to the fetus in the first trimester of pregnancy, and there is no evidence of risk in later trimesters. *Pregnancy Category C: Animal studies have demonstrated that a risk to the fetus exists, although there are no adequate studies in pregnant women, the benefits to the fetus may be acceptable despite the potential risks. Or, there are no animal reproductive studies, and there are no adequate studies in pregnant women. *Pregnancy Category D: Animal studies have demonstrated that a risk to the fetus exists, and there is evidence that there is human fetal risk; the use of the agent in pregnant women may be acceptable despite the potential risks. The use of hte agent in pregnant women should be considered only when the potential risk to the mother outweighs the potential risk to the fetus. *Pregnancy Category X: Studies in animals or humans demonstrate fetal abnormalities, or serious adverse effect reports indicate profound fetal risk. These agents should NOT be used in pregnant women under any circumstances. *Of the antibiotics listed, azithromycin is the only medication in pregnancy category B. *Ciprofloxacin and Clarithromycin are in pregnancy category C. *MY ANSWER: Doxycycline and Sulfamethoazole-trimethoprim are in pregnancy category D. Note: trimethoprim-sulfamethoxazole is considered a pregnancy category B agent in the first trimester and a category D agent in the second and third trimesters. Therefore, keeping in mind the FDA pregnancy categories of each agent, azithromycin would be the most recommended agent for this patient.
A six-year-old child is brought to the pediatrician by his mother. While playing with friends after school, he received a puncture wound on his hand. Although his mother washed and disinfected it to the best of her ability, she is now concerned about the possibility of tetanus, and is seeking medical advice. After checking the child's vaccination status, the physician advised the mother that a new tetanus booster is not necessary at this time. If bacteria have been introduced into this child's puncture wound, which of the following pairs of complementary molecules will be most important in causing phagocytic cells to enter the area of infection?
LFA-1 and ICAM-1
LFA-1 is responsible for strong binding between monocytes, T lymphocytes, macrophages, neutrophils, and dendritic cells, and injured endothelium as well as each other. LFA-1 is a member of the beta-2 integrin family, and interacts with ICAM-1, a member of the Ig superfamily containing five Ig-like domains. This interaction causings the strong adhesion necessary to promote diapedesis into the area of acute inflammation. *MY ANSWER: CD28 and B7 is a set of complementary molecules expressed on T cells and antigen-presenting cells. It plays a role in T cell activation, but not in diapedesis. *CR3 and ICAM-1 is not correct. Although CR3 is an integrin that binds to fibrinogen, iC3b, and ICAM-1, it is not the most important interaction in calling phagocytic cells out of the circulation and into an area of active inflammation. It does promote movement throught the extracellular matrix and encourage phagocytosis. *MAdCAM-1 and L-selectin is an interaction important in homing of lymphocytes to mucosal surfaces. It is believed to be important in the formation of secondary immune aggregates of lymphoid cells in the submucosa along the digestive tract and respiratory tree. *P selectin and adressins are complementary molecules found on endothelium and platelets. They are believed to contribute to the initiation of the clotting process in an area of acute inflammation but are not directly involved in the movement of phagocytes into the tissue spaces.
A 33-year-old single mother of two young children visits her physician because of an oral ulcer. A review of systems is significant for fatigue, myalgia, and joint pain. Laboratory results demonstrate leukopenia, and a high-titered antinuclear antibody. A speckled staining pattern due to anti-Sm is seen with immunofluorescence; urinary protein is elevated. Which of the following is the most likely diagnosis?
Systemic Lupus Erythematosus
Systemic lupus Erythematosus is a prototype connective tissue disease. The diagnosis reauires four criteria to be met from a list of eleven possible criteria: malar rash, discoid rash, photosensitivity, oral ulcers, arthritis, serositis, renal disorder, neurologic disorder, hematologic disorder, immunologic disorder, and antinuclear antibody. The patient also has anti-Sm, which is pathognomonic for SLE, but is only found in 30% of the affected patients. Antinuclear antibodies (ANA) are present in 95-100% of cases of SLE; anti-double-stranded DNA is found in 70% of the cases. *Generalized fatigue due to being a single working mother of two children could well be a possibility, but the presence of the other criteria make SLE more likely. *Goodpasture's syndrome is characterized by linear disposition of immunoglobulin, and often C3, along the glomerular basement membrane. Glomerulonephritis, pulmonary hemorrhage, and occasionally idiopathic pulmonary hemosiderosis occur. *MY ANSWER: Mixed Connective Tissue Disease is an overlap syndrome characterized by a combination of clinical features similar to those of SLE, scleroderma, polymyositis, and rheumatoid arthritis. These patients generally have a positive ANA in virutally 100% of the cases. High titer anti-ribonucleoprotein (RNP) antibodies may be present, generating a speckled ANA pattern. Anti-RNP is not pathognomonic for mixed connective tissue diseasse, since it can be found in low titers in 30% of patients with SLE. *Scleroderma is characterized by thickening of the skin caused by swelling and thickening of fibrous tissue, with eventual atrophy of the epidermis. ANA are often associated with the disease, but the staining pattern is generally nucleolar.
A 60-year-old teacher is in an automobile accident and has cuts and contusions treated in a local emergency department. She is treated and released, but 2 days later seeks the advice of her own physician because one of her facial injuries has become hot, red, and swollen. On examination, she is found to have a temperature of 39 C (102.2 F). There is regional lymph node involvement and evidence of spreading cellulitis from the advancing edge of the infection. An aspirate is taken from the area, and a catalase-negative, gram-positive coccus is cultured. A petechial rash is observed in the area and her fingertips are darkly discolored and cool to the touch. What do the immunologic cells responsible for this pattern of symptoms have in common with each other?
They have similar variable-beta chains in their T cell receptors
This woman is developing the signs of Streptococcus pyogenes exotoxin-mediated toxic shock. Streptococcus pyogenes is on of two gram-positive cocci that is capable of contributing to exotoxin-mediated pathology by virture of its elaboration of exotoxins that act as superantigens. The other organism capable of this is Staphylococcus aureus, with its production of toxic shock syndrome toxin (TSST-1). Superantigens such as these have their effecvt by causing the release of dangerous amounts of IL-1, IL-6, and TNF-alpha from macrophages and interferon-gamma from helper T cells. They do this by cross-linking the MHC class II molecules on antigen-presenting cells and the T cell receptors of large numbers of clones of T cells, causing stimulation of these cells in the absence of antigen specificty. Small amounts of these mediators are beneficial in the local area of an injury, but when their release becomes system-wide, they can be life-threatening. *MY ANSWER: Antigen specificity is NOT shared between the T helper cells and antigen-prsenting cells involved in the production of toxic shock. this is the important difference between the circumstances that provide protective activation of TH cells and macrophages, and those that (here) cause dangerous and life-threatening results. *Complementarity between the T cells receptor and a MHC class II/peptide conjugage is NOT shared between the T helper cells and antigen-presenting cells involved in the production of toxic shock. This is the important difference betweeen the circumstances that provides protective activation of TH cells and macrophages and those that (here) cause dangerous and life-threatening results. *They have phagocytized Streptococcus pyogenes is NOT correct. Toxic shock is caused in the absence of any specific antigen presentation to TH cells. All that is necessary to cause the conjugate formation between the affected TH cells and macrophages is that similarities within clones of T cells in the variable-beta regions of their T cell recptors are bound by the superantigens involved. *They have similar alpha-2 domains of MHC class I molecules is NOT correct. Streptococcus pyogenes erythrogenic exotoxins act as superantigens by cross-linking the variable-beta regions of the T cell receptor of multiple clones of T cells to the class II MHC molecules on large numbers of antigen-presenting cells. This linkage does not involve the class I MHC molecule.