Which of the following is a myeloid/monocyte marker?

• A. CD34
• B. CD3
• C. CD20
• D. CD11b

Answer: (D)

CD11b is a myeloid/monocyte marker because it is part of the Mac-1 integrin complex (with CD18) and is characteristically expressed on cells of the myeloid lineage, including monocytes, macrophages, neutrophils, and some dendritic cells. This makes it a classic indicator of myeloid/monocyte origin in tissue sections and helps distinguish monocyte/macrophage–related infiltrates from lymphoid lineages.

In contrast, other markers point to different lineages: CD34 marks hematopoietic progenitor or blast cells, indicating early precursors rather than mature myeloid cells; CD3 is a T-cell marker; and CD20 marks B cells. Thus, CD11b best identifies myeloid/monocyte populations in immunohistochemistry.