Role of cytokines

Cytokines are small glycoproteins produced by a number of cell types, predominantly leukocytes, that regulate immunity, inflammation and hematopoiesis. They regulate a number of physiological and pathological role including innate immunity, acquired immunity and a plethora of inflammatory responses. The discovery of cytokines was initiated in the 1950s, but the precise identification of their structure and function took many years. The original discoveries were those of IL-I, IFN and nerve growth factors (NGFs); however, these cytokines were purified and given their names years later. Elucidation of the precise physiological, pathological and pharmacological effects of some of the cytokines is still in progress.

The modern techniques of molecular biology were principally responsible for their complete identification and as a consequence, several hundred cytokine proteins and genes have been identified, and the process still continues. Cytokines are produced from various sources during the effector phases of natural and acquired immune responses and regulate immune and inflammatory responses. They are also secreted during nonimmune events and play a role unrelated to the immune response in many tissues. Generally, their secretion is a brief, self-limited event. They not only are produced by multiple diverse cell types, but also act upon many different cell types and tissues. Cytokines often have multiple effects on the same target cell and may induce or inhibit the synthesis and effects of other cytokines. After binding to specific receptors on the cell surface of the target cells, cytokines produce their specific effects. Multiple signals regulate the expression of cytokine receptors. The target cells respond to cytokines by new mRNA and protein synthesis, which results in a specific biological response. They act through receptors, and are especially important in the immune system; cytokines modulate the balance between humoral and cell-based immune responses, and they regulate the maturation, growth, and responsiveness of particular cell populations. Some cytokines enhance or inhibit the action of other cytokines in complex ways.