Both strains of influenza virus could successfully infect MBL WT and MBL KO mice as evidenced by the significant weight loss in these mice after virus infection. Compared to MBL KO mice, MBL WT mice had more weight loss upon virus infection. For pdmH1N1 virus infection, MBL WT mice showed a significantly greater body weight drop on days 3-12 compared with MBL KO mice ( Figure 2B ). The mean peak body weight loss observed in the MBL WT mice and MBL KO mice were -23.51% and -17.38%, respectively. For H9N2/G1 virus infection, MBL WT mice only showed a significantly greater weight loss on day 8 when compared with MBL KO mice ( Figure 2C ). Although similar mean peak weight loss was observed between the MBL WT mice and MBL KO mice after H9N2/G1 virus infection, MBL WT mice recovered more slowly than the KO mice. Collectively, these data suggested that the presence of MBL caused a more severe infection by the pdmH1N1 and H9N2/G1 viruses.