Plaques in a virus plaque assay are
Answer: A
A plaque forms where an infection has spread through neighbouring cells and killed them, leaving a clear patch in an otherwise intact monolayer. Viruses form no colonies, because they do not grow on media, only in cells.
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A plaque assay measures
Answer: B
Each plaque records that a virus was able to enter a cell, replicate, spread to its neighbours and damage them, so the count reflects infectivity rather than particle number. Many particles in a preparation are non-infectious and are invisible to this assay.
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Evidence of viral growth in cell culture is commonly seen as
Answer: C
Damage to the cell sheet, or inclusions visible within cells, signals that a virus is replicating. Turbidity, gas and fermentation reactions are all bacterial phenomena, since they depend on metabolism the virus does not possess.
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In serodiagnosis of a viral infection, the antibody class appearing relatively rapidly after infection is
Answer: D
IgM appears early, so finding it suggests a recent or current infection and can support a diagnosis from one sample. IgG rises later and persists, which is why it is followed across two samples rather than read on its own.
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A rising titre of IgG antibody is demonstrated by testing
Answer: A
Two samples taken at different points in the illness are compared, because a single IgG result cannot distinguish present infection from a past one. The comparison is what makes the rise, rather than the absolute value, the meaningful finding.
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A live attenuated viral vaccine differs from an inactivated one in that it
Answer: B
Because the virus replicates in the recipient, one course often gives immunity that lasts for life. The cost of that advantage is the possibility of reversion to a pathogenic form, which is why such vaccines need extensive testing.
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The principal disadvantage of a live attenuated vaccine is
Answer: C
A live vaccine strain has been weakened by laboratory manipulation, and a weakened organism may mutate back toward virulence. Recombinant methods that delete the regions likely to revert are one way of reducing that risk.
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An inactivated viral vaccine generally
Answer: D
A killed preparation cannot replicate in the recipient, so the immune system meets only the antigen supplied. That makes the vaccine safe from causing infection but means high antigen concentrations and repeat doses are usually needed.
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Interferon, added to normal cells in culture, is described as
Answer: A
Interferon is a protein that puts uninfected cells into a state in which viral replication is resisted, which is why it acts broadly rather than against one agent. Alpha interferon has proved useful clinically in chronic hepatitis B and C.
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How many types of interferon are recognised?
Answer: B
Correct answer: Three: alpha, beta and gamma
Three classes are recognised. Alpha and beta are the type I interferons, produced by many kinds of cell in direct response to viral infection and acting to put neighbouring cells into an antiviral state. Gamma is type I I, produced by activated T cells and natural killer cells, and it is chiefly immunoregulatory rather than directly antiviral.
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