**Core Concept**
Hairy cell leukemia is a rare, chronic lymphoproliferative disorder characterized by the clonal expansion of mature B lymphocytes. The neoplastic cells in hairy cell leukemia exhibit a distinctive "hairy" appearance under light microscopy due to the presence of hair-like projections on their surface.

**Why the Correct Answer is Right**
The neoplastic cells in hairy cell leukemia are of B-cell origin, specifically expressing the B-cell antigen CD22 and the B-cell receptor CD19. These cells also produce large amounts of immunoglobulin M (IgM) and other B-cell markers, confirming their B-cell lineage. The hairy cell leukemia cells are characterized by their unique morphology, including the presence of hair-like projections (virchow-Robin spaces) and the expression of specific B-cell antigens.

**Why Each Wrong Option is Incorrect**
**Option A:** T cells are a type of lymphocyte that plays a central role in cell-mediated immunity, but they are not the primary cell type involved in hairy cell leukemia.
**Option C:** Myeloid cells are a type of hematopoietic cell that gives rise to cells such as neutrophils and monocytes, but they are not the cells involved in hairy cell leukemia.
**Option D:** Macrophages are a type of immune cell involved in the phagocytosis of foreign particles and cellular debris, but they are not the cells that proliferate in hairy cell leukemia.

**Clinical Pearl / High-Yield Fact**
Hairy cell leukemia is often associated with splenomegaly (enlargement of the spleen) and cytopenias (low blood cell counts), particularly anemia and thrombocytopenia. The diagnosis of hairy cell leukemia is typically made by bone marrow biopsy and immunophenotyping.

✓ Correct Answer: B. B. cells