Any of a class of soluble proteins (molecular weight less than 100,000 daltons, or 100 kD) that are released by a cell to send messages that 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).

Unlike cells of the endocrine system, many different types of cells can produce the same cytokine, and a single cytokine may act on a wide variety of target cells. Further, several cytokines may produce the same effect on a target, so the loss of one type of cytokine may have few if any consequences for the organism; this situation is called redundancy. Finally, the response of a target cell may be altered by the context in which it receives a cytokine signal. The context includes other cytokines in the milieu and extracellular matrix. Thus has developed the concept of cytokines as letters of an alphabet that combine to spell words which make up a molecular language.