This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Blood, Subject name: Medicine
Hairy cell leukemia is 
Select and explain. 
A. B cell tumor, B. T cell tumor, C. NK cell tumor, D. All of above
A: B cell tumor
Answer: A: B cell tumor 
Explanation: Ans. (a) B cell tumorRef: Harrison 19th ed. / 706* Hairy cell leukemia is a B cell tumor disease that presents in older males.* Usual presentation involves pancytopenia with splenomegaly.* The malignant cells appear to have "hairy'projections on light and electron microscopy and show a characteristic staining pattern with tartrate-resistant acid phosphatase.* Bone marrow is typically not able to be aspirated, and biopsy shows a pattern of fibrosis.* Patients with this disorder are prone to unusual infections, including infection by Mycobacterium avium intracellulare, and to vasculitic syndromes.* Hairy cell leukemia is responsive to chemotherapy with interferon , pentostatin, or cladribine, with the latter being the usually preferred treatment.