IFNG and chronic obstructive pulmonary disease: Chronic systemic inflammation in COPD may also contribute to neuroinflammatory processes that underlie depression [95], and elevated levels of pro-inflammatory cytokines such as IL1β, IFN-γ, IL-2, and tumor necrosis factor (TNF) have been observed in patients suffering from both COPD and depression [96,97] (Figure 2).