Interleukin 17A (IL-17A) is the founding member of the IL-17 family, which is composed of six members with different patterns of expression. The IL-17 family is characterized by a common disulfide-linked homodimeric structure, and different family members can also form heterodimers, such as IL-17A/IL-17F. IL-17 family members act as pro-inflammatory mediators involved in the immunity against extracellular pathogens, but they are extensively studied mainly for their pathogenic role in autoimmunity and allergy. T helper cells secrete IL-17A after polarization in Tʜ17 cells by specific cytokines such as TGF-β1, IL-6, IL-23, and IL-1β. Both IL-17A and IL-17F act on a specific receptor complex formed by IL-17RA and IL-17RC, expressed on different stromal cells in several tissues, such as endothelial cells, fibroblasts, and immune cells. IL-17A increases chemokine secretion by target cells and recruitment of monocytes and neutrophils. Secretion of IL-17A by Tʜ17 cells is common in several inflammatory and autoimmune diseases, such as rheumatoid arthritis, lupus, psoriasis, and multiple sclerosis. IL-17A also sustains angiogenesis and hematopoietic processes. Inhibitors of IL-17A are investigated as therapeutic tools in autoimmune disorders. Human and murine cells show partial cross-reactivity for corresponding homologous IL-17A.