Cytokines are small signaling molecules secreted by immune white blood cells that have a specific effect on the interactions and communications between cells. They stimulate immune cells to increase (upregulate) or decrease (downregulate) inflammation and affect other immune responses.  Cytokine is a general name for the over thirty-three cytokines that have been identified.  Some cytokines that are involved in immunological function are: tumor necrosis factor (TNF), interferon (IFN), interleukin-2 (IL-2) and interleukin-10 (IL-10).

There are both pro-inflammatory cytokines and anti-inflammatory cytokines. Cytokines may act on the cells that secrete them (autocrine action), on nearby cells (paracrine action), or in some instances on distant cells (endocrine action).