Interleukin-4 (IL-4) is a pleiotropic cytokine that has many biological roles, including the stimulation of activated B-cell and T-cell proliferation, and the differentiation of naïve helper T-cell to Th2 cells. Interleukin-4 is a co-stimulator of DNA-synthesis and is induces the expression of class II MHC molecules on resting B cells.