The family of IL-17 cytokines is one of the most ancient cytokines that plays an important role in host defense. In particular, IL-17 cytokines are critical for cutaneous and mucosal immunity against extracellular pathogen and fungal infections. Cells of both innate and acquired immune arms produce the IL-17 cytokine. However, aberrant production and dysregulation of IL-17 cytokine signaling has been linked to several autoimmune inflammatory diseases. Data from experiments in animal models and humans with genetic defects in the IL-17 pathway tend to suggest that IL-17 cytokines produced by innate cells are more important for host defense, while IL-17 cytokines derived from Th17 cells are associated with inflammation in autoimmune diseases. Nonetheless, therapeutic strategies applied by current global approaches, which is blocking the effector IL-17 cytokine (IL-17A and IL-17F) activity, have achieved remarkable efficacy with an acceptable safety profile. The most impressive efficacy is in treating psoriasis. This may also reflect the nonimmune effects of IL-17 cytokines on keratinocytes. Thus, IL-17A is a potent cytokine stimulating keratinocyte proliferation.