Which cell marker is characteristic of Hairy Cell Leukemia?

Hairy Cell Leukemia is characterized by the presence of tartrate-resistant acid phosphatase (TRAP) activity. This enzyme is often used as a marker to help diagnose the disease. In Hairy Cell Leukemia, the leukemic cells exhibit peculiar cytoplasmic projections that give the cells their "hairy" appearance. The identification of TRAP activity is crucial because it distinguishes Hairy Cell Leukemia from other types of leukemias and lymphomas, particularly those involving B-lymphoid neoplasms.

In addition to TRAP, the leukemic cells in Hairy Cell Leukemia typically express B-cell markers such as CD19 and CD22, but they are negative for several other common markers such as CD5 and CD23. The BCR-ABL fusion protein, often associated with Chronic Myeloid Leukemia (CML), is not relevant in Hairy Cell Leukemia and does not play a role in its pathophysiology. Therefore, TRAP is the definitive marker that identifies and characterizes Hairy Cell Leukemia.