Monocytes are a type of human white blood cell. Normally, monocytes account for 3%-8% of the total white blood cells in peripheral blood. A low monocyte count means that the number of monocytes is below the normal range. The common causes of low monocyte count are: First, bone marrow hematopoietic failure disease, the most common disease is aplastic anemia, due to the immune damage of autologous T lymphocytes to hematopoietic stem cells, resulting in the normal proliferation and division of hematopoietic stem cells, so that the monocyte count will appear low. Second, patients with malignant tumors may have low monocyte count after receiving radiotherapy or chemotherapy due to damage to hematopoietic stem cells by chemotherapy drugs or radiation. Third, as seen in acute lymphoblastic leukemia, low monocyte counts may occur due to the proliferation of leukemia cells in the bone marrow and the impact on normal cell proliferation and division.