A factor secreted by a cell that affects other cells, often as part of an immunological response; e.g. an interleukin or an interferon (but some cytokines are also involved in co-ordinating development, as in embryogenesis). The distinction between cytokines and hormones is somewhat arbitrary. Classic hormones, such as insulin, are generated and released by specific organs or tissues into the general circulation and affect distant organs, in contrast with cytokines, which are released from individual cells at lower concentrations and act locally (but sometimes also systemically).