HID-HSCT Versus IST as First-line Treatment for SAA

Inclusion Criteria: * (1) Confirmed diagnosis of severe aplastic anemia, including very severe aplastic anemia, severe aplastic anemia, and hepatitis-associated severe aplastic anemia. (2) Age 14-70 years. (3) No HLA-matched related donor available. (4) HIV negative, HBV negative, HCV negative. (5) No absolute contraindications to transplantation or immunosuppressive therapy. (6) Signed informed consent form must be obtained before study procedures begin; for subjects aged 18 years or older, informed consent shall be signed by the patient themselves or direct family members. Considering the patient's medical condition, if the patient's own signature is medically inadvisable, the informed consent shall be signed by a legal guardian or the patient's direct family member. Exclusion Criteria: 1. Inherited bone marrow failure syndromes (IBMFS), including Fanconi anemia, dyskeratosis congenita, Shwachman-Diamond syndrome (SDS), etc.; 2. Clonal cytogenetic abnormalities or bone marrow examination suggesting pre-MDS or MDS; 3. Known severe allergy to ATG; 4. Previous allogeneic or autologous hematopoietic stem cell transplantation; 5. Previous solid organ transplantation; 6. Uncontrolled infection at enrollment, or requiring mechanical ventilation or hemodynamic instability; 7. Active HIV replication at enrollment, detectable HCV antibody positivity and HCV-RNA positivity within 90 days prior to enrollment, or HBsAg positivity; known seropositivity for HIV or active hepatitis C virus; 8. History of malignant tumors (except resected basal cell carcinoma or treated cervical carcinoma in situ); 9. Psychiatric disorders or other conditions that prevent compliance with study treatment and monitoring requirements; 10. Inability or unwillingness to sign the consent form; 11. Other special circumstances deemed ineligible by the investigator.