Advances in fish cytokine biology give clues to the evolution of a complex network

Complex immune systems require complex regulatory mechanisms. Whilst the control of innate and inflammatory responses is likely to be ancient in origin, and potentially ubiquitous in the animal kingdom, clearly the evolution of an adaptive immune system in vertebrates is likely to coincide with new regulatory mechanisms and molecules. Key amongst these is the cytokine network, well characterised in mammals and becoming increasingly well characterised in fish and birds. This article will review our current understanding of which components of this network are crucial for adaptive immunity and evolved early, being present from fish to mammals. It will also consider which components of this network potentially evolved later (after the fish-tetrapod divergence) leading to unique cytokine genes in particular vertebrate groups.