IFNG and systemic-onset juvenile idiopathic arthritis: Similar effects occur in macrophage activation syndrome (MAS) complicated with systemic juvenile idiopathic arthritis (sJIA) and secondary hemophagocytic lymphohistiocytosis (6, 7), in which ferritin is highly elevated, associated with the storm of various cytokines, including IL-1, IL-2, IL-6, IL-18, IFN-γ, and others.