OBJECTIVE: Allergic asthma is a common inflammatory disease regulated by the T helper (Th) cells. Interferon regulatory factor 4 (IRF4) plays an important role in the differentiation of Th cells. This study investigated whether IRF4 is involved in the systemic immune responses in allergic asthma patients. METHODS: Peripheral blood mononuclear cells (PBMCs) were isolated from allergic asthmatics (n = 29) and healthy controls (n = 12). The mRNA and protein levels of IRF4 in PBMCs were measured by quantitative RT-PCR and western blotting. The frequencies of Th1, Th2, and Th17 cells were analyzed by flow cytometry. The related cytokine (interferon-γ, interleukin-4 (IL-4), and IL-17) concentrations in plasma and culture supernatants were measured by ELISA. The levels of GATA binding protein 3 (GATA3), retinoic acid-related orphan receptor γt (RORγt), and forkhead box P3 (FOXP3), the master transcription factors controlling Th2, Th17, and T regulatory cells differentiation, respectively, were examined by quantitative RT-PCR. RESULTS: The levels of IRF4 were elevated in allergic asthmatics compared with those in healthy controls. The frequencies of Th2 and Th17 cells as well as the concentrations of Th2- and Th17-related cytokines were higher in plasma from asthma patients than those from healthy controls; similar results were observed in culture supernatants. IRF4 mRNA levels were positively correlated with GATA3, RORγt, and FOXP3 mRNA levels in allergic asthmatics but not in healthy controls. CONCLUSIONS: Our results suggested that IRF4 may play a role in the systemic inflammation of allergic asthma patients by facilitating the differentiation of Th2 and Th17 cells at least at the transcriptional level.