Haemagglutination inhibition antibodies in nasal secretions of persons after natural parainfluenza virus infection.

In 7 adults and 7 children with upper respiratory illness of parainfluenza aetiology, the virus was isolated in the acute phase from nasal washings or nasal smears in spite of high titres of haemagglutination inhibition (HI) antibodies in serum. By contrast, secretory HI antibodies were not demonstrated at the onset of illness in any of the patients, but their formation started early and the antibodies reached maximal levels about 10 days after onset of illness. The individual patients showed considerable differences in the dynamics of secretory antibody formation and especially in their persistence. In some of the patients, secretory antibodies were demonstrated as late as 12 months after the illness.