Which marker is most characteristic of myeloid cells, including monocytes, in IHC panels?

• A. HLA-DR
• B. CD8
• C. Pax-5
• D. CK7

Answer: (A)

The marker most characteristic of myeloid cells, including monocytes, is HLA-DR. This is a class II MHC molecule expressed on antigen-presenting cells, such as monocytes, macrophages, dendritic cells, and activated B cells. In immunohistochemistry, HLA-DR positivity supports myeloid/antigen-presenting lineage and helps distinguish these cells from lymphoid populations driven by markers like CD8 for cytotoxic T cells or Pax-5 for B cells. CK7 marks epithelial cells, not hematopoietic lineages, so it’s not useful for identifying myeloid cells. While CD8 and Pax-5 identify T and B cells respectively, HLA-DR uniquely flags the myeloid/antigen-presenting compartment among the options.