M2e- and NP-specific Ab responses of BALB/c mice convalescent from primary, secondary and tertiary infection. Sera from groups of BALB/c mice, obtained 3–5 weeks after recovery from primary, secondary or tertiary infection by heterotypic or heterosubtypic influenza A viruses, were tested by HeLa-M2 ELISA for M2-specific Ab concentration. Each symbol shows the mean of at least two independent assays, each performed in triplicate or quadruplicate. Cross-bars indicate the geometric mean titer of all groups. B. The same sera were tested for NP-specific Ab titers by ELISA against purified NP.