1 Introduction Severe aplastic anemia (SAA) is a bone marrow failure caused by abnormalities in cellular immunity [1]. The immune mechanisms of aplastic anemia include an imbalance in the Th1/Th2 ratio under unknown antigen stimulation, increased secretion of hematopoietic negative regulators, proliferation, and excessive activation of cytotoxic T lymphocytes (CTLs), which abnormally attack hematopoietic stem/progenitor cells and ultimately lead to bone marrow failure [2-5].
