Decreased bone marrow B-cell chimerism is usually due to immune rejection and may be caused by disease relapse. However, the clinic usually tests only the total bone marrow cell chimerism and less often the B cell chimerism alone. Generally the immune system in the body recognizes non-self components and attacks them to form a rejection reaction. In bone marrow transplantation, if the donor and the recipient are related, the rejection will be weaker, but if the donor and the recipient are not related, the rejection may be more severe. After bone marrow transplantation, if the degree of bone marrow B cell chimerism decreases, indicating that the immune effect of the recipient’s cells is stronger, while the immune effect of the donor’s cells is weaker, the dosage of immunosuppressant drugs such as tacrolimus can be reduced to alleviate the suppression of the immune effect of the donor’s cells. In addition, if donor cell chimerism decreases after transplantation in patients with acute leukemia or myelodysplastic syndromes, it may be related to relapse of the disease, and targeted treatment such as lymphocyte infusion from the original donor may be indicated. In the event of a decrease in bone marrow B-cell chimerism, it is recommended that treatment be standardized under the guidance of a physician to avoid delaying the disease. In addition, the above drugs should also be used under the guidance of a doctor.