Inflammatory cytokines are key in the so-called sickness response, which features signs and symptoms that greatly overlap with the clinical presentation of CFS and FM [49] A recent large study exploring inflammatory markers in patients with CFS reported dysregulation of many cytokines, such as TGF-β, IFN-γ, IL-4, IL-13, and IL-17F [27]. Here, IL4 is linked to myalgic encephalomeyelitis/chronic fatigue syndrome.