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. The gene discussed is IL1B; the disease is macrophage activation syndrome.