Hemagglutination by influenza viruses can be inhibited by antibody to hemagglutinin or, under certain conditions, by antibody to neuraminidase. In the hemagglutination-inhibition (HI) test with antibody to hemagglutinin the HI titer is approximately proportional to the reciprocal of the virus concentration. In contrast, in the neuraminidase-hemagglutination inhibition (N-HI) test a 2-fold increase in virus dosage produced a nearly 7-fold decrease in antibody titer. The addition of antihuman IgG

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... in most instances vastly increased the N-HI titers. The magnitude of enhancement was strongly dependent upon the combination of human sera, the virus dose and the particular lot of antihuman IgG. Fifteen of 21 commercial lots of antihuman IgG produced an enhancing effect. Greatest enhancement (up to a 1000-fold) was more often produced by antisera with the highest precipitating antibody potency (E-value) to human IgG. The enhancement of N-HI titers by antiantibody constitutes a highly sensitive test system. Each reactant is a potentially major variable which may appreciably alter the test results.