Hairy cell leukemia (HCL) is a rare, indolent B-cell lymphoproliferative disorder characterized by accumulation of mature B lymphocytes with distinctive cytoplasmic projections ('hairy' appearance) in the bone marrow, spleen, and peripheral blood. The hallmark genetic mutation is BRAF V600E (present in nearly 100% of classic HCL), which constitutively activates the MAPK/ERK signaling pathway, promoting malignant cell survival and proliferation. HCL cells infiltrate the bone marrow causing reticulin fibrosis (leading to 'dry tap' on bone marrow aspiration), resulting in pancytopenia — anemia, neutropenia, and thrombocytopenia. Massive splenic infiltration causes splenomegaly, the most common physical finding. The distinctive immunophenotype includes CD20+, CD22+, CD25+, CD103+, CD11c+, and annexin A1+ (most specific marker). Unlike other leukemias, HCL typically presents with pancytopenia rather than leukocytosis. Treatment with purine analogs (cladribine or pentostatin) achieves complete remission in 80–90% of patients, making HCL one of the most treatable adult leukemias.