Interleukin-4 (interleukin-4, IL-4) is a cytokine secreted by type II helper T cells (Th2 cells). The biological effects of interleukin-4 include stimulation of activated B cell and T cell proliferation, and differentiation of CD4+ T cells into type II helper T cells. It also plays a key role in regulating humoral immunity and adaptive immunity. Interleukin-4 induces B cell antibody class switching to IgE, upregulating the production of a second type of major tissue-compatibility complex. It is similar to a mouse protein B cell stimulating factor-1 (BCSF-1).