Abstract: Objective To explore the effect of particulate matter ≤2.5 microns in aerodynamic diameter (PM_2.5) on expressions of inflammatory cytokines in respiratory damage in rats.Methods Ninety Wistar rats were randomly divided into 9 experimental groups exposed to low-,moderate-,and high-dose of PM_2.5 (via trachea injection of PM_2.5 saline suspension) and polluted air (one day after PM_2.5 exposure) containing low-,moderate-,and high-content of sulfur dioxide (SO₂),nitrogen dioxide (NO₂),and carbon monoxide (CO) and 3 control groups treated with normal saline and normal air;the treatments were conducted 4 hours a day continuously for 1,7,and 30 days.The inflammatory cytokines in bronchoalveolar lavage fluid (BALF) were detected with enzyme-linked immunosobent assay (ELISA) and the mRNA of inflammatory cytokines in lung tissues were detected with real-time fluorescent quantitative PCR.Results Compared with those in the control groups,the expressions of interleukin-4 (IL-4),interleukin-6 (IL-6),and tumor necrosis factor-α (TNF-α) in BALF and their mRNA in lung tissues were increased first and then decreased in experimental groups during the treatments of PM_2.5 and air pollutants;while the level of interferon gamma (IFN-γ) showed no significant change,with a declining trend.Conclusion Humoral immunity is a major response in the process of respiratory system injury caused by airborne PM_2.5 in rats with unbalanced T helper 1 (Th1)/T helper 2(Th2)ratio.PM_2.5 could not activate protective immune response of Th1 and macrophages in lung could not eliminate PM_2.5 effectively.