Neutrophils but not eosinophils express Siglec-9. Freshly isolated mature blood and immature bone marrow neutrophils, as well as blood monocytes and eosinophils, were analyzed by flow cytometry. (A) Neutrophils and monocytes, but not eosinophils, demonstrated evidence for Siglec-9 expression (red). A significant proportion of monocytic Siglec-9 was additionally detectable after sialidase treatment (blue). (B) A subpopulation of the bone marrow neutrophils was Siglec-9 positive (red). Two-color immunofluorescence analysis demonstrated a correlation between CD11b and Siglec-9 expression. CD11b is a differentiation marker associated with myelocytes, metamyelocytes, and mature neutrophils. Representative examples of 4 independent experiments are shown. Staining with isotype-matched control mAb is indicated by black lines in the histograms. In panel B, middle and right subpanels, the quantitative analysis is shown by percent at the top right corner of each dot plot.