As a result of immunological studies of the last 25 years it turned out that interleukin-7 (IL-7) is one of the most important regulatory cytokines of the immune system. It is special in many concerns. Having a strong impact on the development, proliferation and activation of immune cells it is, however, cannot be attributed to the classic cytokine-activators, like IL-2, as many types of cells require its presence at almost all stages of the development, and its spectrum of activities is impressive. The review aims to summarise the numerous data obtained on the IL-7 structure, biological activity, biological significance and features of functioning studied both in vitro and in vivo.We made the main emphasis on the IL-7 signal transduction pathways that determine the fate of T and B cells, as well as some other cells important for homeostasis of the organism. In our opinion, the analysis of literature in this field not only reveals a host of questions still unanswered, but also makes the path for perspective investigation more visible.
