Avidity of IgG antibodies against mumps, parainfluenza 2 and Newcastle disease viruses after mumps infection.

Paired serum samples from 39 patients with recent mumps infection were assayed for IgG antibodies against mumps, parainfluenza 2 and Newcastle disease virus (NDV). A modified enzyme immunoassay was used, giving separate estimates of high avidity antibodies (EHAA) and total specific antibodies (ETSA). A marked cross-reaction was seen between mumps and parainfluenza 2 virus, with changes of ETSA between paired samples of about the same magnitude against both these viruses. The mean change of EHAA against mumps was, however, significantly greater than that against parainfluenza 2. There were 16 patients who had a change of ETSA greater against parainfluenza 2 than against mumps. When the EHAA responses were compared, there were only 8 such patients. The responses against NDV were negligible. Estimation of antibody avidity, even by the arbitrary method used, can distinguish between homotypic and cross-reactive heterotypic antibodies after mumps infection. The implications for expressing the results of enzyme immunoassay are discussed.