IL-4 binding to cellular IL-4R and its inhibition by soluble IL-4R (sIL-4R). (a) IL-4 binds to cellular IL-4R, mediating cellular activation and important pro-inflammatory functions in asthma. For clarity, only the IL-4Rα chain of the heterodimer is shown, represented by three domains: extracellular (green), transmembrane (white) and cytoplasmic (red). It should be noted, however, that both chains of the heterodimer are required to initiate intracellular signaling. (b) The sIL-4R consists of the extracellular portion of IL-4Rα. It retains the ability to bind IL-4 with high affinity and high specificity and thereby functions as a decoy receptor which can sequester naturally occurring IL-4 and prevent it from activating the cell. sIL-4R cannot activate cellular signaling; thus, it serves as an anti-inflammatory agent that can counter the effects of IL-4 in asthma.