IFNG and rheumatoid arthritis: Increased levels of several pro-inflammatory cytokines (tumor necrosis factor (TNF)-α; interleukin (IL)-1α, IL-1β, IL-5, IL-6, IL-8, IL-13, IL-15, and IL-21; and interferon (IFN)-γ) in smokers with systemic autoimmune diseases including RA have also been well-documented [74,90,91,92,93,94,95].