Cytokines are small secreted proteins that regulate immunity, inflammation, and cell growth. They are produced by white blood cells and tissue macrophages in response to immune stimuli. The main classes of cytokines include interferons, interleukins, tumor necrosis factors, growth factors, and chemokines. Interferons are produced during viral infections and help limit viral spread. Interleukins are involved in cell communication and direct cell division. Tumor necrosis factors can induce cell death in tumor cells.