Hairy cell leukemia is a rare lymphoid neoplasm arising from mature B-lymphocytes. Clinically, the disease presents with splenomegaly and abdominal discomfort, frequent infections, fatigue and bleeding because of related cytopenias. Bone marrow biopsy is essential for diagnosis. Below we describe a case of a 70-year-old African-American male who presented to our hematology clinic complaining of fatigue. Clinical exam and computed tomography imaging did not reveal splenic enlargement. Blood work-up revealed pancytopenia and bone marrow was diagnostic for hairy cell leukemia.The patient was started on cladribine, with gradual improvement of his symptoms and blood count abnormalities. Therefore, it is essential to keep hairy cell leukemia in the differential of pancytopenia even in the absence of a splenomegaly. 1. Introduction Hairy cell leukemia (HCL) is a rare lymphoproliferative disorder originating from B-lymphocytes and accounts for less than 1% of lymphoid malignancies in The United States [1]. The disease’s name is derived from characteristic hairy cytoplasmic projections of malignant B-lymphocytes. It is relevant to mention that the disease is more common among people of Caucasian and Ashkenazi Jewish-descent [2]. The pathogenesis of HCL is not completely understood. However, several pathologic factors play a role in the development of this disease. First, mutations in BRAF proto-oncogene were found to be implicated in the pathogenesis of HCL [3]. Overactivation of BRAF pathway leads to uncontrolled cellular proliferation. In addition to that, HCL cells produce various cytokines such as basic fibroblast growth factor, transforming growth factor, and tumor necrosis factor alpha which lead to fibrosis and suppression of the bone marrow with resultant pancytopenia [4]. Some patients with HCL may have abnormalities of fifth chromosome [4]. It is interesting to note that the malignant cell of HCL is a mature B-lymphocyte at a late stage of development (up to preplasma cell) [4]. These cells express cluster of differentiation (CD) antigens characteristic of a mature B-lymphocyte such as CD 19, CD 20, and CD 22 as well as CD 11c, CD 25, and CD 103 [5]. HCL cells do not possess CD antigens of B-lymphocytes at earlier stages of development such as CD 10 and CD 21 and plasma cells such as CD 5 and CD 23 [6, 7]. Therefore, it is believed that hairy cells are clonal Blymphocytes arrested at a late stage of development [2]. Clinical presentation of HCL is nonspecific and is related to cytopenias [6]. Affected patients may complain of fatigue, easy and
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