CXCL8 and myalgic encephalomeyelitis/chronic fatigue syndrome: To detect signs of chronic low-grade inflammation and evaluate respective markers in ME/CFS patients, pro-inflammatory and mucosal barrier integrity-related cytokines IFN-gamma, TNF-alpha, IL-6, IL-1-beta, IL-8, and IL-33 were measured in serum samples, as well as complement factor protein C4a and FGF21, which are exported into the circulation by the liver in response to stress.