The 6H6 monoclonal antibody specifically recognizes the Interleukin-3 receptor alpha chain (IL-3Ra) which is also known as CD123. IL-3Ra (CD123) is a ~70 kDa type I transmembrane glycoprotein that is encoded by IL3RA (interleukin 3 receptor subunit alpha) which belongs to the type I cytokine receptor family within the immunoglobulin gene superfamily. This receptor chain consists of an extracellular region that contains an immunoglobulin-like N-terminal domain (NTD) with a fibronectin type III (FnIII) fold followed by two more FnIII domains that form the cytokine receptor module (CRM), a transmembrane region, and an intracellular tail. IL-3Ra (CD123) binds IL-3 specifically and with low affinity. IL-3Ra (CD123) forms a high-affinity signaling receptor for IL-3 (IL-3R) with the ß common chain (ßc; also known as, CD131) that is shared with the heterodimeric IL-5 and granulocyte-macrophage colony-stimulating factor (GM-CSF) receptors. IL-3Ra (CD123) is variably expressed on certain hematopoietic progenitor cells, basophils, eosinophils, mast cells, monocytes, macrophages, dendritic cells, megakaryocytes, and on some B cells, endothelial cells, and leukemia cells. Bound IL-3 can signal through IL-3R to promote the activation, proliferation, differentiation, and viability of these cells. Amongst monoclonal antibodies specific for human IL-3Ra (CD123), the 6H6 and 9F5 antibodies do not block IL-3 binding to the IL-3R whereas the 7G3 antibody does block IL-3 binding to its receptor in a dose-dependent manner.