There are a plethora of cytokines and we keep finding new ones all the time. They are small polypeptide molecules less than 80kilodaltons in weight that may or may not be glycosylated. They have very important roles in a whole range of body functions from general maintenance through regulating inflammatory responses to directly interfering with invading pathogens. Cytokines are produced in large amounts by many cells but are very potent even at minute concentrations. Their action is usually limited to affecting other cells in the local area of their production but they can also have an overall systemic effect acting much like hormones.
Cytokines act on other cells by bonding to cytokine receptors on the surface of cells.
We assume that each cytokine has its own particular cytokine receptor, but only some of the cytokine receptors have been identified so far. The difficulty in finding cytokine receptors is that there are so few of them on each cell, between 10 and 10,000. With so few it is difficult to purify them and identify their structure. Cytokine binding to cell receptors can induce a significant change in cell activity and production of proteins. Different cytokines have different actions on the same cell.