Influenza A (H1N1) is the greatest pandemic threat in 2009 [9] . The clinical course of H1N1 can be severe, particularly for very young patients with complications. The immune response affected by pathological damage to the body caused by S-OVI and clinical prognosis. Studies have shown that the incidence of patients with severe H1N1 was highest in the 15-to 19-year-old age group and the lowest for patients aged 65 or above [10] . Our clinical data also showed that the most severe infections occur in individuals younger than 25 years old or in middle-aged patients Figure 1 Analysis of the dynamic changes in the lymphocyte subgroup and C-reactive protein (CRP) in the peripheral blood of patients with Influenza A (H1N1) 2009. CD4 + T, CD8 + T, CD4 + CD25 + Treg, NK, and CD19 + B cells are presented as a percentage of total peripheral blood leukocytes (PBLs) in severe H1N1 (n = 36) patients, moderate H1N1 (n = 40) patients, and healthy volunteers (n = 20). †: denotes statistically significant differences compared with severe H1N1; *: denotes statistically significant differences compared with the healthy volunteers; **: denotes statistically significant differences compared with the moderate H1N1 patients and with the healthy volunteers.