The frequency of peripheral blood CD19 + B cell count of patients with severe H1N1 gradually increased within the first 3-14 d of treatment. The NK cell count showed a gradual decline, whereas the T cell subtypes (CD4 + CD25 + , CD4 + , and CD8 + T cells) showed no significant changes. The frequency of peripheral CD4 + T and CD4 + CD25 + Treg cells of patients with moderate H1N1 were significantly higher than in patients with severe H1N1 within the first 1-2 d of treatment. The NK cell counts of patients with moderate H1N1 were similar to that of patients with severe H1N1, which demonstrated a gradual decline, whereas the frequency of CD19 + B and CD8 + T cells showed no significant changes.