M2e- and NP-specific Ab titers in human sera from the acute and convalescent phase of naturally acquired influenza virus infection. A and B. Paired sera from the acute and convalescent phase of 24 patients were tested in ELISA against HeLa-M2. M2-specific Ab titers were quantified by comparison to the binding seen with purified M2e-specific mouse-human chimeric Ab TRF-3.1. A. Sera from 11 patients that showed a ≥ 4-fold increase in M2-specific Ab titer between acute and convalescent phase. Each line connects acute and convalescent titers of an individual patient. B. Sera from 13 patients that showed <4-fold increase between acute and convalescent phase. The stipulated horizontal line in panels A and B indicate the threshold of detection. C and D. The same sera were tested in ELISA for NP-specific Ab titers. The titers are expressed relative to one convalescent serum sample with a high NP Ab titer, which was assigned an arbitrary value of 100. C and D show the acute and convalescent NP-specific titers of the same patients shown in A and B, respectively.