The mIL4R-M2 monoclonal antibody specifically binds to the α subunit of the mouse Interleukin-4 Receptor (IL-4Rα, aka, CD124). The mIL4R-M2 antibody does not recognize human or rat IL-4R; crossreactivities with IL-4R from other species have not been established. The mouse IL-4Rα is a 140 kDa transmembrane glycoprotein that is expressed by B and T lymphocytes and a variety of other hematopoietic and nonhematopoietic cells and cell lines. The cell surface IL-4Rα chain binds IL-4 with high affinity and associates with either the common γ chain (IL-4Rα/γc; aka, type I IL-4R) or the IL-13 receptor alpha subunit (IL-4Rα/IL-13Rα; aka, type II IL-4R complex) to form two distinct types of signal-transducing IL-4R complexes. The type I IL-4 receptor complex specifically binds IL-4 whereas the type II IL-4R binds and transduces signals from either IL-4 or IL-13. The mIL4R-M2 antibody blocks IL-4 binding to cells and is reported to be a potent inhibitor of IL-4's biological activities. The mIL4R-M2 antibody also recognizes naturally occurring, soluble truncated forms of IL-4Rα (sIL-4R) that result either from enzymatic cleavage of the cell surface extracellular IL-4Rα domain or from differential mRNA splicing and secretion by cells. These sIL-4R retain their high-affinity ligand binding domain and appear to either enhance or inhibit IL-4-mediated functions depending on the relative local levels of IL-4 and sIL-4R. The immunogen used to generate the mIL4R-M2 hybridoma was CTLL-19.4 cells, a subclone of CTLL-2 cells that express high levels of IL-4Rα.