Human CD123, the 70 kd IL-3 receptor α chain (IL-3Rα), is associated with the 120-140 kd β subunit. The β chain is shared with the receptors for interleukins IL-5 and GM-CSF. IL-3Rα is expressed on hematopoietic progenitors and plays an important role in hematopoietic progenitor cell growth and differentiation. This antibody has been reported to block the binding of 125I-IL-3 to high and low affinity IL-3 receptors. In functional experiments, this antibody was found to inhibit acute myeloid leukemia cell proliferation, basophil histamine release, endothelial cell-mediated IL-8 secretion, and neutrophil transmigration. At the Fifth HLDA Workshop, the human IL-3 receptor was designated CD123. This antibody reacts with plasmacytoid dendritic cells (PDC). PDC plays a crucial role in the initiation of antiviral and antitumoral immune responses and produce type I interferons a and b. CD123 antibody is useful in diagnosis of Kikuchi disease, hyaline vascular type of Castleman disease, lupus, and primary cutaneous marginal zone B-cell lymphoma.