Macrophages and neutrophils account for the highest percentage of the total lung leukocytes following infection with highly pathogenic influenza viruses We next determined specific immune cell sub-populations given by percent of the total lung leukocyte population in order to reveal differential population dynamics amongst the immune cell populations measured in these studies (Table 2) . Strikingly, macrophage populations by day 3 pi, represented 24% and 24.4% (nearly one-quarter) of the total gated lung leukocytes of mice infected with 1918 and Thai/16 (HP) viruses, respectively, which was significantly higher than the frequency of macrophages detected in TX/91 and SP/83 virus-infected mice from days 2 though 9 p.i. (^p,0.005). Neutrophil populations were elevated in all infection groups compared to mock levels beginning 1 day p.i ( Figure 2 , panel B) however percentages of neutrophils in HP virus infected mice were significantly higher (* p,0.05) than those in LP virus-infected mice beginning day 2 p.i. and levels remained elevated at each subsequent time point measured ( Table 2 ).