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The CD34 antigen is a single-chain transmembrane glycoprotein, Mr 105 to 120 kilodaltons (kd). The antigen is associated with human hematopoietic progenitor cells and is a differentiation stage-specific leucocyte antigen. Clone My10 and clone 8G12 recognize two distinct CD34 epitopes; at least three epitopes have been identified.The CD34 antigen is present on immature hematopoietic precursor cells and all hematopoietic colony-forming cells in bone marrow and blood, including unipotent (CFU-GM, BFU-E) and pluripotent progenitors (CFU-GEMM, CFU-Mix, and CFU-Blast). Terminal deoxynucleotidyl transferase-positive B- and T-lymphoid precursors in normal bone marrow are CD34 . The CD34 antigen is present on early myeloid cells that express the CD33 antigen but lack the CD14 and CD15 antigens and on early erythroid cells that express the CD71 antigen and dimly express the CD45 antigen. The CD34 antigen is also found on capillary endothelial cells and approximately 1% of human thymocytes. Normal peripheral blood lymphocytes, monocytes, granulocytes, and platelets do not express the CD34 antigen. CD34 antigen density is highest on early hematopoietic progenitor cells and decreases as cells mature. The antigen is absent on fully differentiated hematopoietic cells. Uncommitted CD34 progenitor cells are CD38- and lack lineage-specific antigens such as CD71, CD33, CD10, and CD5, while CD34 cells that are lineage-committed express the CD38 antigen in high density. Most CD34 cells reciprocally express either the CD45RO or CD45RA antigens, with the CD45RO population being the more primitive.Approximately 60% of acute B-lymphoid leukemias and acute myeloid leukemias (AMLs) and 1% to 5% of acute T-lymphoid leukemias express the CD34 antigen. The antigen is not expressed on chronic lymphoid leukemias or lymphomas.
