Cytokines are a family of small proteins that mediate an organism's response to injury or infection. Cytokines operate by transmitting signals between cells in an organism. Minute quantities of cytokines are secreted, each by a single cell type, and regulate functions in other cells by binding with specific receptors. Their interactions with the receptors produce secondary signals that inhibit or enhance the action of certain genes within the cell. Unlike endocrine hormones, which can act throughout the body, most cytokines act locally, near the cells that produced them.

Cytokines are crucial to an organism's self-defense. Cells under attack release a class of cytokines known as chemokines. Chemokines participate in a process called chemotaxis, signaling white blood cells to migrate toward the threatened region. Other cytokines induce the white blood cells to produce inflammation, emitting toxins to kill pathogens and enzymes to digest both the invaders and the injured...