Serum IgM antibody and influenza A infection.

Sucrose density gradient ultracentrifugation followed by haemagglutination inhibition for demonstrating specific influenza IgM was evaluated as a means of confirming recent infection with influenza A viruses. Specific IgM antibodies were found in at least one serum obtained from 83% of patients with proven recent infection with influenza A viruses but in none of the sera from 21 individuals without evidence of infection. Influenza IgM antibodies persisted for up to 112 days after infection. The relative merits of detecting specific IgM and complement fixing antibodies for diagnostic purposes are discussed.