IFNG and chronic obstructive pulmonary disease: Further investigation demonstrated that before COPD patients had received medical treatment, the IL-18 had positive correlation with IFN-γ (P = 0.001), IL-1 (P = 0.024), and TNF-α (P = 0.020) and negative correlation with IL-10 (P = 0.003); besides, it had no correlation with other cytokines (Figure 1(c)); the result indicated that IL-18 had great connection with IFN-γ.