1. An assay in which a dilute solution of a virus is applied to a culture of the host cells and the number of plaques (areas of dead or transformed cells) that develop indicates the number of infective viruses (plaque-forming units, pfu) in the solution. Secondary infection by convective spread is inhibited by making the medium viscous.
2. An assay for the number of cells producing antibody against erythrocytes or against antigen bound to the erythrocytes. A clear plaque of haemolysis surrounds cells that secrete antibody (plaque-forming cells).
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