GVHD Prophylaxis by Addition of CD20 Monoclonal Antibody to the Conditioning Regimen in SAA With Treatment of Allo-HSCT

Inclusion Criteria: * Subjects eligible for inclusion in this study must meet all of the following criteria: 1. SAA characterized Bone marrow cellularity\< 25%, or 25-50% with \<30% residual hematopoietic cells and pancytopenia, with at least two of the following parameters in peripheral blood Absolute neutrophil count \< 0.5\*10E9/L Platelet count \< 20\*10E9/L Absolute reticulocyte count \< 20\*10E9/L 2. ALL patients will undergo allo-HSCT. 3. Subjects aged \<50 years old with KPS performance status ≥70 at the same time. 4. Aspartate aminotransferase (AST) , alanine aminotransferase (ALT) and alkaline phosphatase≤2 times the upper limit of normal (ULN). Blood urea nitrogen and Creatinine ≤1.25 times ULN. 5. Cardiac function of subjects must meet all of the following requirements: ECG examination do not reveal any acute myocardial infarction, arrhythmia, or first-degree or higher atrioventricular block. No signs of heart failure. No carrying of active rheumatoid heart disease. Chest radiograph or physical examination do not indicate an enlarged heart. 6. ALL subjects show none contraindication for allogeneic hematopoietic stem cell transplantation. 7. Patients enrolled in the rituximab group have no contraindications for the use of rituximab. 8. Patients and their clients are willing to perform hematopoietic stem cell transplantation. 9. Potential donor is accessible. 10. Patients have no anti-HLA antibodies. Exclusion Criteria: 1. Subject who is unable comprehend or is unwilling to sign an informed consent form or consent form due to severe physical or mental illness resulting in a survival of less than 2 years. 2. Presence of clinically active uncontrolled significant chronic infections (including bacterial, fungal or viral infection), such as dental caries, otitis media, sinusitis, etc., need to be carried out after effective control. 3. Past medical history of severe pulmonary dysfunction. 4. Past medical history of diabetes with a propensity for ketoacidosis. 5. Presence of severe coagulopathy, thrombophlebitis or pulmonary embolism. 6. Presence of decompensated liver insufficiency or active hepatitis. 7. Presence of history of severe autoimmune disease. 8. Past medical history of thyroid dysfunction with currently abnormal thyroid function. 9. Any concomitant malignancies that have not been disease-free for 5 years. 10. Past medical history of hypersensitivity to biological products (including antibiotics). 11. Pregnant or nursing woman. 12. Inherited bone marrow failure.