Innate immunity represents the first line of host defense against extracellular infections. Timely initiation of inflammatory responses to invading pathogens confers host the ability to prevent, restrict, and eliminate microorganisms and further trigger adaptive immunity to provide extended eradication of the pathogens as well as long-term protection of the host from recurrent infections. During this process, various types of cytokines are required for efficient immune responses, and interleukin 17 (IL-17) is one of them. The IL-17 family of cytokines bridges innate and adaptive immune responses and play an essential role in defining the Th17 cell lineage differentiation and function. Th17 cells have pivotal roles in protecting the host against extracellular pathogens as well as in promoting inflammatory pathology in autoimmune diseases. Strict regulation of the IL-17 family of cytokines at different levels ensures appropriate controls of their abilities to shape immune responses. This chapter provides the recent progress in the field of IL-17 studies for the understanding of the regulatory mechanisms in the expression and signaling of the IL-17 family cytokines. IL-17A (IL-17) is the most well-studied member in this family, while information on the other members is relatively sketchy. Thus, we mainly focus on the study of IL-17A.
