Interleukin-4 (IL-4) exhibits many biological and immunoregulatory functions on B lymphocytes, monocytes, dendritic cells and fibroblasts. The IL-4 gene is located on chromosome 5 and displays several cell-specific regulatory sequences in its promoter, which explain its restricted secretion pattern to activated T cells and mast cells. The IL-4 receptor is multimeric and is constituted by at least IL-4Rα, a chain common to other cytokine receptors. Two types of IL-4 receptors have been defined: one constituted by the IL-4Rα and the γc chain, and a second constituted by the IL-4Rα and the IL-13Rα, which is able to transduce both IL-4 and IL-13 signals. Major events of IL-4 transducing signal have now been elucidated and are known to be mediated through JAK/IRS-2 and STAT6 pathways. Numerous studies have also demonstrated the key regulatory role of IL-4 in allergic responses as well as its anti-tumor and anti-inflammatory effects.