2.1  Patients
AA cases and myelodysplastic syndrome (MDS) cases were all from Shanghai Children’s Hospital between. According to the severity, AA patients were classified into non-severe AA, severe AA and very severe AA7. MDS was diagnosed based on WHO classification criteria8. Twenty children with solid tumors without bone marrow infiltration and any hematological complications were used as normal controls. Bone marrow biopsy specimens from AA, MDS and normal controls were used for immunohistochemistry. The bone marrow fluid of the first suction and peripheral blood from AA and controls were detected by flow cytometry.