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. Here, IL33 is linked to myalgic encephalomeyelitis/chronic fatigue syndrome.