Simultaneous determination of influenza virus antihemagglutinin and antineuraminidase antibodies by means of the combined photometric ACU method

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Abstract
A modification of the photometric ACU (antibody concentration unit) method designated as combined ACU test is described by means of which the titers of both antihemagglutinin (AH) and antineuraminidase (AN) antibodies can be simultaneously determined, using intact homologous influenza virus. In addition, the constants describing the reactions of AH and AN antibodies with their antigens can be obtained. These constants were determined for a series of H2N2 and H3N2 influenza virus strains. This paper gives a detailed description of the conduct of test. The sensitivity and accuracy of AH antibody titration by means of the combined ACU test was found to be the same as achieved by means of the original ACU test. The same conclusion was reached when comparing the titration of AN antibodies by means of the combined ACU test and of a modified ACU test which, in turn, has been found to be more sensitive and accurate than is AN antibody titration by the enzyme inhibition test. However, the sensitivity of AN antibody titration by the combined ACU test was found to decrease progressively as the ratio of AH to AN antibodies exceeded certain limits.