The first system of the complement fixation reaction consists of
Answer: D
Correct answer: an antigen, an antibody, complement
The first system is the reaction being tested: antigen, antibody and complement. If the antigen and antibody correspond, the complex fixes the complement and uses it up; if not, the complement is left free.
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Immunofluorescence test is a
Answer: D
Correct answer: serological reaction with the use of labels
Immunofluorescence is a labelled antibody technique: the antibody carries a fluorescent dye, and binding is read under a fluorescence microscope. It belongs with ELISA and radioimmunoassay, which use enzyme and isotope labels, rather than with the unlabelled reactions of precipitation, agglutination and lysis.
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Erythrocytes sensitized with antigens are called
Answer: A
Correct answer: erythrocytic diagnosticum
Red cells coated with a known antigen make an erythrocytic diagnosticum: they clump visibly when the corresponding antibody is present, so a reaction too fine to see becomes a plain agglutination. The technique is indirect (passive) haemagglutination, and the red cell serves only as an inert carrier.
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Endos medium is a
Answer: A
Correct answer: differential media
Endo medium contains lactose and a fuchsin-sulphite indicator, so lactose fermenters such as E. coli grow as dark red colonies with a metallic sheen while non-fermenters stay colourless. Distinguishing organisms by their appearance on the plate is what makes a medium differential.
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During the lag phase
Answer: A
Correct answer: Microorganisms increase in size of cell and metabolic rate
In the lag phase there is no increase in cell number: the organisms are adapting to the new medium, synthesising the enzymes and intermediates they need. The cells grow larger and their metabolic rate rises, and only then does division begin.
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Which component of the gram-negative cell wall is responsible for endotoxic activity ?
Answer: B
Correct answer: Lipid A of the lipopolysaccharide
Lipid A anchors the lipopolysaccharide in the outer membrane and is the portion that triggers fever, hypotension and shock when released. The O-antigen determines serological type, and lipoteichoic acid belongs to gram-positive walls.
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Patient serum contains
Answer: D
Correct answer: known antibody
In the usual serological test the patient’s serum is the source of antibody - and because the antigen used is known, an antibody detected by it is identified, that is, known. The unknown quantity is whether it is present and at what titre.
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What is Chemotaxis?
Answer: C
Correct answer: In the presence of a chemical compound, swimming towards or away of a bacteria
Chemotaxis is movement in response to a chemical gradient - towards an attractant such as a sugar or amino acid, and away from a repellent. The cell cannot sense a gradient across its own length, so it compares concentrations over time, lengthening its runs when conditions improve and tumbling when they worsen.
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Which of the following is true about fungi
Answer: A
Correct answer: Eukaryotes
Fungi are eukaryotes: true nucleus, mitochondria, 80S ribosomes and a wall of chitin. Most are free-living saprophytes and grow perfectly well on cell-free media - which is exactly what distinguishes them from the obligate intracellular parasites.
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Which of the following statements about the Fc fragment of an antibody is not true?
Answer: A
Correct answer: posses antigen combining activity
Antigen binding is the work of the Fab fragments, whose variable regions form the binding sites. The Fc fragment is the constant stem, and it carries the effector functions instead: complement fixation, placental transfer of IgG, and binding to cells - skin fixation by IgE, and opsonisation through Fc receptors on phagocytes.
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