Hairy Cell Leukemia

Hairy cell leukemia (HCL) is a rare, indolent mature B-cell malignancy. Abnormal B lymphocytes accumulate in the bone marrow, spleen, and sometimes peripheral blood. Their fine cytoplasmic projections create the characteristic “hairy” appearance on microscopy. Marrow infiltration and fibrosis reduce normal blood-cell production and explain the central clinical pattern: - Reduced red-cell production causes fatigue, pallor, exertional dyspnea, dizziness, or tachycardia. - Neutropenia increases susceptibility to infection, and the inflammatory response may be muted. - Thrombocytopenia increases petechiae, bruising, mucosal bleeding, and major hemorrhage risk. - Monocytopenia is characteristic and can strengthen recognition of the overall pattern. Fibrosis can make the marrow difficult to aspirate. A dry aspirate therefore does not mean the marrow is empty; a core biopsy can demonstrate the infiltrated, fibrotic marrow. The spleen often enlarges, producing early satiety, left-upper-quadrant fullness, or discomfort. Pancytopenia or monocytopenia with splenomegaly is more typical than marked leukocytosis. Prominent bulky lymphadenopathy is unusual in untreated classic HCL and should prompt the hematology team to reconsider the diagnostic category. Classic HCL is closely associated with the BRAF V600E pathogenic variant. Flow cytometry...