Serum IgM and IgA responses in influenza A infections.

Immunofluorescent serum IgM and/or a significant level of IgA antibody was detected in 87% of 39 cases of current or recent influenza A infection from two to 84 days after the onset of illness. Secondary IgM staining occurred in 5% of sera and a significant correlation was found between complement-fixing and class-specific antibodies. It was estimated that the immunofluorescent test could be diagnostic in 64% of single sera with levels of CF antibody between 32 and 256, and because anti-s antibody was detected in the IgM test this test did not differentiate primary and secondary influenza A infection.