The frequency of the different lymphocyte subsets at different periods were statistically compared and the results show that the frequency of peripheral CD4 + and CD4 + CD25 + T cells in the moderate H1N1 patients were higher than that of the severe H1N1 patients within the first 1-2 d (Figures 1a and 1c ; P < 0.05). The CD19 + B cell counts of the severe H1N1 patients were significantly higher than those of the moderate H1N1 patients and the control group for the same period (Figure 1e) , whereas the NK cell counts were significantly less than that of the control group (Figure 1d ; P < 0.05). The frequency of CD8 + T cells had no significant difference (P > 0.05; Figure 1b) among the three groups.