Interleukin-12 (IL-12) is a cytokine, a type of signaling protein produced by immune cells. It plays a crucial role in the immune response by promoting the activity of T cells and natural killer cells, which are essential for fighting infections and tumors. IL-12 helps to enhance the body's ability to respond to pathogens and can influence the development of T helper 1 (Th1) cells, which are important for cell-mediated immunity. IL-12 is composed of two subunits, p35 and p40, which combine to form the active cytokine. It is primarily produced by macrophages and dendritic cells in response to infections. Research into IL-12 has shown its potential in immunotherapy, particularly in treating certain cancers and chronic infections, by boosting the immune system's response.