Interleukin 12 p40 subunit (IL-12p40) is a component of the bioactive cytokines IL-12 and IL-23. Existing in either a monomeric or homodimeric form it is induced in excess of the other subunits of IL-12 and IL-23. Most known for providing negative feedback by competing for the IL-12 receptor it also functions as a chemoattractant for macrophages and is associated with several pathogenic inflammatory responses.