Interleukin 5 (IL-5) is an interleukin produced by type-2 T helper cells and mast cells. Through binding to the interleukin-5 receptor, interleukin 5 stimulates B cell growth and increases immunoglobulin secretion—primarily IgA. It is also a key mediator in eosinophil activation. IL-5 is a 115-amino acid (in human, 133 in the mouse) -long Th2 cytokine that is part of the hematopoietic family. Unlike other members of this cytokine family (namely interleukin 3 and GM-CSF), this glycoprotein in its active form is a homodimer. The IL-5 gene is located on chromosome 11 in the mouse, and chromosome 5 in humans, in close proximity to the genes encoding IL-3, IL-4, and granulocyte-macrophage colony-stimulating factor (GM-CSF), which are often co-expressed in Th2 cells. IL-5 is also expressed by eosinophils and has been observed in the mast cells of asthmatic airways by immunohistochemistry.[5] IL-5 expression is regulated by several transcription factors including GATA3.