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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 tissue. If the inflammatory response is not enough to deal with the problem, additional immune system cells are also summoned by cytokines to continue the fight.

In a serious injury or infection, cytokines may call the hematopoietic, or blood-forming system into play. New white blood cells are created to augment the immune response, while additional red blood cells replace any that have been lost. Ruptured blood vessels emit chemokines to attract platelets, the element of the blood that fosters clotting. Cytokines are also responsible for signaling the nervous system to increase the organism's metabolic level, bringing on a fever that inhibits the proliferation of pathogens while boosting the action of the immune system.

Because of the central role of cytokines in fighting infection, they are being studied in an effort to find better treatments for diseases such as AIDS . Some have shown promise as therapeutic agents, but their usefulness is limited by the tendency of cytokines to act locally. This means that their short amino acid chains are likely either to be destroyed by enzymes in the bloodstream or tissues before reaching their destination, or to act on other cells with unintended consequences.

Other approaches to developing therapies based on research into cytokines involve studying their receptor sites on target cells. If a molecule could be developed that would bind to the receptor site of a specific cytokine, it could elicit the desired action from the cell, and might be more durable in the bloodstream or have other advantages over the native cytokine. Alternatively, a drug that blocked receptor sites could potentially prevent the uncontrolled inflammatory responses seen in certain autoimmune diseases.

See also Autoimmunity and autoimmune diseases; Immunochemistry; Immunodeficiency disease syndromes; Immunodeficiency diseases