Normal myeloid antigen expression and relationships in bone marrow. (A-B) SSC, FSC, and CD45 intensity in marrow cells from a healthy donor. Promyelocytes contain primary granules giving rise to the high SSC signal. Later stage neutrophils are marked by a decrease in SSC related to the appearance of secondary granules. (C-D) CD11b and HLA-DR show a characteristic relationship in normal maturing myeloid cells and monocytes that follow maturation stages. Normal monocytes retain HLA-DR throughout their development in contrast to neutrophils that rapidly lose this antigen at the promyelocyte stage. The maturation of monocytes is first identified by the rapid appearance of CD11b while maintaining intermediate levels of CD45. (E-F) CD13 is a unique antigen that appears, disappears, and reappears relatively quickly during normal myeloid development. Normal myeloblasts and promyelocytes express high levels of CD13. The antigen is lost on myelocytes and then gradually emerges again as the cells develop to the segmented neutrophil stages. CD13 and CD16 are normally expressed in a consistent “concave” pattern as shown in panel E. Normal monocytes show heterogeneous coexpression of CD13 and CD16. (G-H) Progression from the myeloblast stage to more mature myeloid cells involves acquisition of high levels of CD15 with loss of CD34 and HLA-DR, whereas mature monocytes maintain dim expression of CD15 and lack CD34. (I-J) As myeloid cells mature, there is a slight decrease in CD33 intensity. Early stages of monocyte development are accompanied by increases in both CD13 and CD33 expression. Later stages of monocyte development are defined by the coordinated increase in both CD45 and CD14. (K-L) Maturing myeloid cells and monocytes are normally negative for CD56 and CD7.