In patients with HLH–MAS, a significant increase of several pro‐inflammatory cytokines, including IL‐1β, IL‐6, IL‐18, TNF‐α (produced by monocyte‐macrophages), IL‐2, and IFN‐γ, of lymphocytic origin, together with hyperferritinemia, cytopenias (mainly thrombocytopenia), and impaired liver function is observed. This evidence concerns the gene IL6 and macrophage activation syndrome.