Cytokine

A cytokine is a small protein that one cell releases to send a message to other cells, or sometimes back to itself. Cytokines are made by many immune cells, such as macrophages and T cells, and also by some cells outside the immune system. They work by binding to matching receptors on target cells, which alters the target's gene expression and behavior.

The main families include interleukins, which pass signals between white blood cells; interferons, which infected cells release to warn their neighbors about a virus; chemokines, which draw particular white blood cells toward an infection; tumor necrosis factor; and colony-stimulating factors, which promote the production of blood cells. A cytokine may act on the cell that made it (autocrine signaling), on nearby cells (paracrine signaling), or travel in the blood to distant targets (endocrine signaling). Some cytokines boost immune activity and others damp it down, and the balance between them shapes inflammation, fever and the transition from innate to adaptive immunity.

Cytokines released during infection cause much of the fever, tiredness and aches that people feel when ill. Several, including some interferons and interleukins, are produced in the lab and used to treat cancer and other diseases.