IL2RA and isolated hyperferritinemia: Currently, HPS is defined by meeting at least five of the following nine diagnostic criteria: (I) fever ≥ 38.5 °C; (II) splenomegaly; (III) cytopenia in almost two hematopoietic lineages (Hb < 9 g/dL, platelets < 100 × 103/μL, absolute neutrophil count < 100/μL); (IV) hypertriglyceridemia (≥3.0 mmol/L); (V) hypofibrinogenemia (≤150 mg/dL); (VI) hyperferritinemia (≥500 μg/L); (VII) elevated soluble levels of IL-2Rα/CD25 (>2400 U/mL); (VIII) histopathological evidence of hemophagocytosis in bone marrow, spleen, or lymph nodes biopsies; and (IX) reduced or absent NK cell activity [13,14].