Circulating cytokines: the tip of the iceberg?

Circulating cytokines, particularly TNF alpha, IL-1, IL-6 and IL-8, can be detected during severe infections consecutively to an exacerbate release by activated cells. The presence or absence of cytokines within biological fluids reflects a rather complex balance between enhancing and inhibitory signals acting on producer cells, between production and catabolism, and between their binding to the target cells and the modulation of their receptors on the cell surface. Furthermore, their presence does not necessarily parallel their activity and a possible interplay between the cytokines and their corresponding inhibitors should be considered. On the contrary, the absence of detectable circulating cytokines does not indicate an absence of production by activated cells. We discuss the concept that circulating cytokines represent the tip of the iceberg.