Lee PY, Schulert GS, Canna SW, et al. - In this study experts examined adenosine deaminase 2 (ADA2), a protein of unknown function released predominantly by monocytes and macrophages, as a novel biomarker of MAS. They included a total of 324 healthy children and adults to establish age-based normal ranges of peripheral blood ADA2 activity. These ranges were compared with 173 children with inflammatory and immune-mediated diseases, including systemic and non-systemic JIA, Kawasaki disease, paediatric systemic lupus erythematosus and juvenile dermatomyositis. These findings collectively distinguish the benefit of plasma ADA2 activity as a biomarker of MAS and allow additional support to a pivotal role of macrophage activation in this condition.