Which marker is expressed on monocytes and polymorphonuclear cells?

The marker that is primarily expressed on monocytes and polymorphonuclear cells is CD14. This protein acts as a co-receptor for the detection of bacterial lipopolysaccharides, facilitating the immune response to pathogens. It is predominantly found on the surface of monocytes, where it plays a crucial role in their activation and the modulation of the immune response. CD14 is also expressed on some subsets of granulocytes, making it a key marker for identifying these cell types in flow cytometry analysis.

In contrast, the other markers have distinct and different roles or cellular distributions. CD25 is the interleukin-2 receptor alpha chain and is primarily expressed on activated T cells and regulatory T cells. CD20 is a pan-B-cell marker important for B-cell maturation and function, while CD34 is a stem cell marker predominantly found on hematopoietic stem cells and endothelial cells, not specifically on monocytes or polymorphonuclear cells. Therefore, CD14 is the only marker listed that fits the criteria of being expressed on both monocytes and polymorphonuclear cells.