SJIA shows a strong association with macrophage activation syndrome (MAS), a form of reactive hemophagocytic lymphohistiocytosis (HLH), characterised by an uncontrolled activation of well-differentiated macrophages releasing a high amount of proinflammatory cytokines, particularly IL-18, which belongs to the IL-1 family. This evidence concerns the gene IL1B and systemic-onset juvenile idiopathic arthritis.