Human antibody to influenza C virus: its age-related distribution and distinction from receptor analogs

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Abstract
Sera from persons of 4 age groups (1-2 yr, 2-5 yr, 20-30 yr and 65-85 yr) were analyzed for hemagglutination inhibition (HI) activity for influenza C virus. Significant HI activity was found in 66% of the 237 sera tested and titers ranged from 8-512. In the young adult group, 96% had antibody and the highest mean titer (74.7) of any age group. Positive sera were far less common in young children (36-47%) and relatively low titers (18.3) were common among adults over 65. The high percentage of sera with antibody to influenza C virus suggests that infections with this virus occur at a rate greater than previously recognized. The high percentage of young adults with elevated levels of HI antibody suggested that an immune response to influenza C infections is common or that the observed HI activity might be attributable, in part at least, to nonspecific inhibitors in the sera. Directly and indirectly, it was shown that most if not all of the inhibitory activity in the human sera examined was due to specific antibody, mostly IgG. The single serum protein fraction with HI activity was found to have a MW equivalent to that of 7S antibody (150,000) and the HI activity was removed by absorption to staphylococcal protein A. Ig from only HI-positive sera bound specifically to cells infected with influenza C virus, as shown by inhibition of hemadsorption and immunofluorescence. These findings were supported by similar results obtained with chicken antisera to C virus.