白细胞减少是循环白细胞（WBC）的降低到<4000/mcl（9/L). It is usually characterized by a reduced number of circulating neutrophils, although a reduced number of lymphocytes, monocytes, eosinophils, or basophils may also contribute. Thus, immune function can be generally decreased.
Platelets are circulating cell fragments that function in the clotting system. Thrombopoietin helps control the number of circulating platelets by stimulating the bone marrow to produce megakaryocytes, which in turn shed platelets from their cytoplasm. Thrombopoietin is produced in the liver at a constant rate and its circulating level is determined by how much is bound to circulating platelets and possibly to bone marrow megakaryocytes and the extent to which circulating platelets are cleared. Platelets circulate for 7 to 10 days. About one third are always transiently sequestered in the spleen.