Definition of "cytokine"

Thus, in the case of viral infection, mechanisms of resistance would be threefold, namely, interferon production, the immune response, including both antibody and lymphokine production, and the generation of lymphokine-like substances by the infected cells themselves. These latter substances have been defined as cytokines.

1975 July, Bigazzi PE, Yoshida T, Ward PA, Cohen S., “Production of Lymphokine-Like Factors (Cytokines) by Simian Virus 40-Infected and Simian Virus 40-Transformed Cell”, in The American Journal of Pathology, volume 80, number 1, page 69

A fundamental feature expressed by the vast majority of cytokines is a profound immunomodulatory activity.Many cytokines, presently available in pure recombinant form, modify bone cell metabolism.

1991, Maxine Gowen, Cytokines and Bone Metabolism, page 26

The cytokines are generally considered to be a group of peptides secreted by cells of the immune system such as macrophages, lymphocytes and T cells, although the same peptides may also be secreted by non-immune cells such as neurological tissues and adipocytes.

2012, Victor R. Preedy, Ross Hunter, editors, Preface: Cytokines, page v

The cytokines provide a classic example of products of the immune system which alter brain and endocrine activities. A variety of cytokines, including interleukin 1, interleukin 2, interleukin 6, and tumor necrosis factor α, have been traditionally associated with peripheral control of the immune system, inflammation, and acute phase response.

2013, Errol B. De Souza, editor, Preface: Neurobiology of Cytokines: Methods in Neurosciences, volume 16, page xiii