any of a group of small, short-lived proteins that are released by one cell to regulate the function of another cell, thereby serving as intercellular chemical messengers. Cytokines effect changes in cellular behaviour that are important in a number of physiological processes, including reproduction, growth and development, and injury repair. However, they are probably best known for the roles they play in the immune system's defense against disease-causing organisms.

As part of the immune response, cytokines exert their influence over various white blood cells (leukocytes (leukocyte)), including lymphocytes, granulocytes, monocytes, and macrophages. Cytokines produced by leukocytes are sometimes called interleukins, while those produced by lymphocytes may be referred to as lymphokines.

Cytokines typically are not stored within the cell but instead are synthesized “on demand,” often in response to another cytokine. Once secreted, the cytokine binds to a specific protein molecule, called a receptor, on the surface of the target cell, an event that triggers a signaling cascade inside that cell. The signal ultimately reaches the nucleus, where the effects of the cytokine are manifested in changes in gene transcription and protein expression—i.e., genes, which code for proteins, may be turned on or off, and protein production may be stimulated or inhibited.

Many different cytokines have been identified, and their activities, at least in part, are known. In some cases, one cytokine can interact with a variety of different cell types and elicit different responses from each cell. In other cases, different cytokines can elicit the same response from a cell. Some cytokines are known to induce or augment the activities of other cytokines, and sometimes their interactions occur through a cascading effect; however, the regulation of and cooperation between these various chemical signals still remain unclear in many cases. The classification of cytokines is problematic because so much remains to be learned about them, but they can be divided into five categories: interleukins (interleukin), interferons (interferon), colony-stimulating factors, tumour necrosis factors (tumour necrosis factor), and growth factors.

Because cytokines are known to play a role in many disease processes, they have the potential to be used in treating a variety of disorders. For example, clinicians monitor levels of cytokines in the blood to assess the progression and activity of certain inflammatory states, such as septic shock. Measuring cytokine production also is useful in determining an individual's immunocompetence, or ability to fight off infection. Cytokines are used as therapeutic agents in treating persons with cancer and immunodeficiency disorders and those undergoing organ transplantation. Cytokines in conjunction with certain vaccines (vaccine) can enhance the vaccines' effectiveness.