Hairy Cell Leukemia

Hairy cell leukemia (HCL) is a rare, indolent B-cell lymphoproliferative disorder. Abnormal mature B lymphocytes accumulate in the bone marrow, spleen, and sometimes peripheral blood. The cells have fine cytoplasmic projections that create the “hairy” appearance on microscopy. The marrow infiltration explains much of the clinical picture. HCL cells stimulate reticulin fibrosis, so the marrow becomes difficult to aspirate; a “dry tap” may result even when the marrow is heavily involved. Normal blood-cell production then falls: - Reduced red-cell production causes fatigue, pallor, exertional dyspnea, or tachycardia. - Reduced neutrophils impair the first-line response to bacterial and fungal infection. - Reduced platelets increase the risk of petechiae, bruising, mucosal bleeding, and hemorrhage. - Monocytopenia is particularly characteristic and can help distinguish HCL from other causes of pancytopenia. The spleen often enlarges because it becomes a major site of malignant-cell accumulation and blood-cell sequestration. A patient may report early satiety, left-upper-quadrant fullness, or discomfort. Marked leukocytosis is not the usual pattern; pancytopenia with splenomegaly is more suggestive. Classic HCL is strongly associated with the BRAF V600E mutation, which persistently activates the...