1.Definition Hematopoietic stem cell transplantation: refers to the application of healthy hematopoietic stem cells to rebuild the patient’s blood and immune system. 2. Classification According to the source of hematopoietic stem cells, hematopoietic stem cell transplantation can be divided into the following categories: bone marrow transplantation, peripheral blood hematopoietic stem cell transplantation, and umbilical cord blood transplantation. Hematopoietic stem cells can be taken from bone marrow, peripheral blood or placental cord blood (referred to as cord blood), and those taken from bone marrow are called bone marrow transplantation, those from peripheral blood are called peripheral blood stem cell transplantation, and those from placental cord blood are called placental cord blood transplantation (referred to as cord blood transplantation). HSCT can be divided into the following categories according to the HLA matching result between the donor and the recipient (patient): Allogeneic HSCT: transplantation between identical twins (twins). Allogeneic hematopoietic stem cell transplantation: Its donors are other people who are not identical twins, including: HLA-matched (referred to as matched) sibling donors, matched semi-matched related donors, and unrelated donors. Transplantation from unrelated donors is also referred to as transplantation from unrelated donors. Autologous hematopoietic stem cell transplantation: The patient’s own hematopoietic stem cells are first frozen and preserved, and then transfused back to the patient after the patient receives high-dose chemotherapy and/or radiation therapy to rebuild his own blood and immune cells, so autologous hematopoietic stem cell transplantation is actually “bone marrow refuge” or “hematopoietic stem cell refuge. Therefore, autologous HSCT is actually “bone marrow refuge” or “hematopoietic stem cell refuge”. Autologous HSCT is usually indicated for patients who have no bone marrow involvement or whose disease is in a good state of complete remission.