Any of numerous hormonelike, low-molecular-weight proteins, secreted by various cell types, which regulate the intensity and duration of immune response and mediate cell-to-cell communication. See: interferon, interleukin, lymphokine, chemokines. See: various growth factors. See Also: interferon, interleukin, lymphokine.
Most cytokines are small (less than 30 kD) soluble proteins or glycoproteins. Produced by macrophages, B and T lymphocytes, mast cells, endothelial cells, fibroblasts, and stromal cells of the spleen, thymus, and bone marrow, they act nonenzymatically through specific receptors to regulate immune responses, in particular modulating the balance between humoral and cell-mediated responses. They are involved in mediating immune and allergic responses by regulating the maturation, growth, and responsiveness of particular cell populations, sometimes including the cells that produce them (autocrine activity). A given cytokine may be produced by more than one type of cell. Some cytokines enhance or inhibit the action of other cytokines. Their complex synergistic and antagonistic interactions fully justify the expression cytokine network. The first cytokines to be identified were named according to their functions (e.g., T-cell growth factor), but this nomenclature became awkward because several cytokines can have the same function, and the function of a cytokine can vary with the circumstances of its elaboration. Later, as the chemical structure of each cytokine was determined, it was designated an interleukin and assigned a number (e.g., interleukin-2 [IL-2], formerly T-cell growth factor). Cytokines have been implicated in the generation and recall of long-term memory and the focusing of attention. Some degenerative effects of aging may be due to a progressive loss of regulatory capacity by cytokines. Because cytokines derived from the immune system (immunokines) are cytotoxic, they have been used against certain types of cancer. Their clinical usefulness is limited by their short half-life and their wide-ranging and unpredictable side-effects.
Reference: Stedman's Medical Dictionary