Structural homology has been able to partially distinguish between cytokines that do not demonstrate a considerable degree of redundancy so that they can be classified into four types:
- The four α-helix bundle family - Member cytokines have three-dimensional structures with four bundles of α-helices. This family in turn is divided into three sub-families:
- the IL-2 subfamily
- the interferon (IFN) subfamily
- the IL-10 subfamily.
- The first of these three subfamilies is the largest. It contains several non-immunological cytokines including erythropoietin (EPO) and thrombopoietin (THPO). Also, four α-helix bundle cytokines can be grouped into ''long-chain'' and ''short-chain'' cytokines.
- the IL-1 family, which primarily includes IL-1 and IL-18
- the IL-17 family, which has yet to be completely characterized, though member cytokines have a specific effect in promoting proliferation of T-cells that cause cytotoxic effects
A classification that proves more useful in clinical and experimental practice divides immunological cytokines into those that enhance cytokine responses, type 1 (IFN-γ, TGF-β, etc.), and type 2 (IL-4, IL-10, IL-13, etc.), which favor antibody responses.
A key focus of interest has been that cytokines in one of these two sub-sets tend to inhibit the effects of those in the other. Dysregulation of this tendency is under intensive study for its possible role in the pathogenesis of autoimmune disorders.
Several inflammatory cytokines are induced by oxidant stress. The fact that cytokines, themselves trigger the release of other cytokines and lead also to increased oxidant stress, makes them important in chronic inflamma
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