Blood circulation levels of IL-18 are elevated in both patients with SJIA and AOSD (Shimizu et al., 2013; Jelusic et al., 2007; Girard et al., 2016), and significantly increased during episodes of MAS (Shiga et al., 2021; Krei et al., 2021); this significant elevation in IL-18 diagnostically distinguishes MAS flares from underlying rheumatic disease (Weiss et al., 2018). Here, IL18 is linked to macrophage activation syndrome.