Low white blood cells and red blood cells both mean that the number of both white blood cells and red blood cells are below the normal range. The most common cause of this condition is a problem with the hematopoietic function of the bone marrow. For example, when the hematopoietic function of the bone marrow fails, both white blood cells and red blood cells drop due to a decrease in the ability of hematopoietic stem cells to proliferate and divide, and the ability to produce blood cells decreases, and one of the most common clinical conditions is aplastic anemia. If the hematopoietic cells of bone marrow show premature apoptosis and pathological hematopoiesis, normal red blood cells and white blood cells cannot develop and mature, and both white blood cells and red blood cells will also decline, and this disease is myelodysplastic syndrome. In addition, after patients with malignant tumors receive chemotherapy or radiation therapy, the damage to hematopoietic stem cells caused by these factors leads to the reduction of hematopoietic stem cells’ ability to proliferate and divide, which also leads to the decline of both white and red blood cells.