1. Any protein secreted by lymph cells that affects cellular activity and controls inflammation.
2. Any of several regulatory proteins; such as, the interleukins and lymphokines, that are released by cells of the immune system and act as intercellular mediators in the generation of an immune response.
3. Any of a group of soluble proteins that are released by a cell to send messages which are delivered to the same cell (autocrine), an adjacent cell (paracrine), or a distant cell (endocrine).

The cytokine binds to a specific receptor and causes a change in function or in development (differentiation) of the target cell.

Cytokines are involved in reproduction, growth and development, normal homeostatic regulation, response to injury and repair, blood clotting, and host resistance (immunity and tolerance).