Innate and adaptive immune responses play critical roles in the body´s defense against viruses. We investigated the host immune response against the 2009 pandemic H1N1 influenza virus [A(H1N1)pdm09] in patients before and after anti-influenza therapy and found that the numbers of dendritic cells and T cells were significantly reduced compared with those of a healthy control group. In contrast, the frequency of natural killer, γδT and T regulatory (T_reg) cells increased, and the concentrations of plasma interferon (IFN)-α/γ and interleukin (IL-15) were significantly higher than those of the control. Following therapy the frequency of γδT and T_reg cells returned to normal; the counts of myeloid dendritic and plasmacytoid dendritic cells were still lower than the control, while the concentrations of IFN-α/γ and IL-15 remained high. We show that infection with A (H1N1)pdm09 was accompanied by changes in peripheral blood lymphocyte subgroups and cytokine profiles, leading to deleterious imbalances in innate and adaptive immunity