Chapter 65 The only lymphoproliferative disorder than can be diagnosed definitively by immunophenotype is acute leukemia expressing CD34. CD34+ cells are precursor cells whose normal counterpart is found in the bone marrow. CD34 is expressed on both myeloid and lymphoid precursors. In most cases of CD34+ acute leukemia no antigens are expressed on the cell surface that would further allow for the definitive distinction between B, T, and myeloid lineages. If such a distinction is considered clinically relevant, the cells can be permeabilized and stained for intracellular expression of CD3 (for T-cell leukemia), Pax5 (for B-cell leukemia), or myeloperoxidase protein (for myeloid origin). The cells in these cases generally will be described by the cytopathologist as immature cells or blasts but occasionally can have a more mature appearance. Almost all cases of CD34+ leukemia show evidence of bone marrow involvement (anemia, thrombocytopenia, neutropenia) and can also show involvement of lymph nodes and spleen as well as a mediastinal mass at presentation. Regardless of the presentation and lineage designation, this form of acute leukemia has been shown to have a poor prognosis (Villiers et al, 2006; Williams et al, 2008).
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