All-Trans Retinoic Acid for the Treatment of Hemophagocytic Lymphohistiocytosis

* Inclusion Criteria 1. Age ≥18 years. 2. Patients diagnosed with active hemophagocytic lymphohistiocytosis according to HLH-2004 diagnostic criteria or the investigator's clinical assessment. 3. Newly diagnosed or treatment-naïve active HLH requiring initial HLH-directed therapy. 4. Presence of active disease manifestations, including at least one of the following: persistent fever, cytopenia, hyperferritinemia, splenomegaly, hypofibrinogenemia and/or hypertriglyceridemia, elevated soluble CD25, hemophagocytosis, reduced or absent NK-cell activity, or other HLH-related organ involvement. 5. Eastern Cooperative Oncology Group performance status of 0-3, or performance status considered acceptable by the investigator in the context of active HLH. 6. Adequate ability to receive oral medication, or ability to receive ATRA through an appropriate enteral route. 7. Expected survival of more than 48 hours in the judgment of the investigator. 8. Female patients of childbearing potential and male patients with partners of childbearing potential must agree to use effective contraception during treatment and for an appropriate period after the last dose of ATRA. 9. Ability to understand and willingness to sign written informed consent. For patients unable to provide consent because of disease severity, consent may be provided by a legally authorized representative according to local regulations. * Exclusion Criteria 1. Prior systemic HLH-directed therapy for the current HLH episode, including etoposide-based therapy, ruxolitinib, emapalumab, alemtuzumab, PD-1 blockade, or other investigational HLH-directed treatment. Short-term corticosteroids, supportive care, anti-infective therapy, or emergency treatment before enrollment may be allowed at the investigator's discretion. 2. Known hypersensitivity to all-trans retinoic acid, tretinoin, retinoids, or any component of the study drug. 3. Pregnant or breastfeeding women. 4. Patients with acute promyelocytic leukemia or other diseases for which ATRA is being used as standard leukemia-directed therapy. 5. Severe uncontrolled infection, shock, respiratory failure, bleeding, or organ failure that, in the investigator's judgment, would make participation unsafe or prevent assessment of study treatment. 6. Severe hepatic dysfunction not primarily attributed to HLH, such as total bilirubin or transaminase levels considered unsafe for ATRA administration by the investigator. 7. Severe renal dysfunction requiring dialysis before enrollment, unless considered related to HLH and acceptable by the investigator. 8. Active intracranial hypertension, pseudotumor cerebri, or uncontrolled severe neurologic disease that may increase the risk of ATRA-related toxicity. 9. Uncontrolled hypertriglyceridemia or other metabolic abnormality that, in the investigator's judgment, would make ATRA treatment unsafe. 10. Concomitant use of vitamin A supplements, other systemic retinoids, or medications with unacceptable interaction risk that cannot be discontinued. 11. Any other condition that, in the investigator's judgment, would interfere with patient safety, protocol compliance, or interpretation of study results.