Low red blood cells, white blood cells, and platelets means that the number of red blood cells, white blood cells, and platelets are all below the normal range, which is clinically called pancytopenia. Pancytopenia can be caused by a variety of diseases, commonly seen: i. Aplastic anemia, which is one of the most common diseases causing pancytopenia. As a result of bone marrow hematopoietic failure, patients develop allogeneic cytopenia. According to the severity of the disease, aplastic anemia is divided into two types: chronic aplastic anemia and acute aplastic anemia. Second, allogeneic cytopenia is also seen in myelodysplastic syndrome, a malignant clonal disorder of hematopoietic stem cells with clinical features of pathological hematopoiesis and high risk of transformation to leukemia, often manifested as a reduction of the three lines in peripheral blood. Severe megaloblastic anemia or severe iron deficiency anemia may also present with reduced trilineage in the peripheral blood.