The IL-6 family of cytokines includes a variety of proteins that function not only within the immune system, but also in other organs, tissues, and types of cells, including neurons. The common evolutionary origin of the IL-6 family proteins determines similar mechanisms of reception and intracellular signaling, although their primary structures are highly variable, as well as their biological functions. We have demonstrated that the members of the IL-6 family have high evolutionary plasticity. This manifests in a high degree of population polymorphism for IL-6 family genes, as well as varying degrees of evolutionary conservation among members of the family. The degree of evolutionary conservation of IL-6 family proteins does not correlate with the mechanisms of interaction between these cytokines and their receptors.