CD8A and Splenomegaly: MAS is characterized by an unregulated immune response with hyper-expansion of CD8+ T cells and uncontrolled macrophage activation.[3] MAS shares similar clinical and pathophysiological features with AOSD such as high fever, splenomegaly, hyperferritinemia, and excessive natural killer (NK) cell and CD8+ T cell activity.[4] Pulmonary arterial hypertension (PAH) is a disease caused by vasoconstriction and vascular remodeling, and manifest with pulmonary artery resistance elevation and high pressure.