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Cytokines are proteins that are involved in cell communication and mediating immune system processes. They represent a diverse group of molecules but mainly growth factors and hormones and falling into one of two broad categories: Type I cytokines include interleukins and colony‐stimulating factors, and Type II cytokines that are typically interferons. Erythropoietin, thrombopoietin, growth hormone, and prolactin have similar structures and signaling mechanisms as Type I cytokines.¹⁵

Improperly regulated cytokines can result in a variety of diseases such as autoimmunity and cancer. These characteristics make them attractive targets for therapeutic purposes. For their function, cytokines need to bind to specific receptors. As such, drugs that target the cytokine‐mediated immune system can include antibodies that neutralize the cytokine or cytokine receptors, recombinant proteins that are receptor agonists or antagonists, or false receptors that will bind the cytokine itself and neutralize it.¹⁶

Cytokines are well studied in animal models that are beneficial for developing therapeutic targets; however, there are limitations due to their natural (native) properties. Cytokines have overlapping activity meaning that, when one cytokine is blocked, another may make up for the lost activity. They are also multifunctional in that they affect several processes, potentially in multiple organs in parallel, so affecting or disrupting a specific cytokine may result in unwanted side effects. Inhibiting the natural activity of cytokines can also result in a severely blunted immune system. Some key characteristics of cytokine type of drugs are provided in Table 3.5.