Animal studies have found that CD4 + T, CD8 + T, and CD19 + B cells achieved peak values within 5-6 d after S-OVI infection, whereas the CD4 + CD25 + Treg cell count reached its peak at 24-48 h after infection [2] . Currently, the use of flow cytometric analysis to measure peripheral blood CD4 + , CD8 + , and CD4 + CD25 + T cells is a simple and common method. Reportedly, the mostly CD4 + CD25 + T cells were also positive for FoxP3 [14] . In this experiment, the peak values for all lymphocyte subgroups in patients with severe H1N1 could not be detected. At least two mechanisms could account for the lower effector T cell response detected after the S-OVI infection, and the peak response time for the effector T cells possibly missed. In addition, the virus could have also led to the immunotolerance of the T cells [15] .