Decades of related research have identified several important findings [10,29,30,31]: (1) The immune response itself, not the underlying diseases or triggering factors, causes the disease pathology; (2) MAS is an immune activation, rather than autoimmunity involving self-antigens; (3) the cytolytic dysfunction of CD8+ T cells and natural killer (NK) cells, rather than macrophages, is a key event in the pathogenesis of MAS; and (4) interferon gamma (IFN-γ), produced by CD8+ T cells, is a key mediator of MAS development. The gene discussed is IFNG; the disease is macrophage activation syndrome.