MAS is believed to be characterized by inappropriate and sustained activation of cytotoxic cells (such as CD8+ T cells and natural killer cells) as well as myeloid cells (macrophages, dendritic cells), leading to a systemic cytokine storm including interferon-γ (IFN-γ), Interleukin-18 (IL-18), Tumor necrosis factor-α (TNF-α), IL-6, IL-1, etc. (3–6). Here, IL1A is linked to macrophage activation syndrome.