TNF and chronic obstructive pulmonary disease: Several biomarkers of systemic inflammation have been proved to be directly associated with COPD, including C-reactive protein (CRP), interleukin-6 (IL-6), interleukin-8 (IL-8), serum amyloid A (SAA), tumor necrosis factor-α (TNF-α), fibrinogen (Fib), and white blood cell (WBC).