Interleukin-17 (IL-17) is a cytokine that plays a dual role in mammalian immunity, acting both as a host defender against infections and as a pro-inflammatory cytokine in autoimmune diseases. The induction of new gene expression by IL-17 is a key mechanism through which it exerts its effects. IL-17 targets a wide range of cells, including mesenchymal and epithelial cells, and induces the production of various cytokines, chemokines, inflammatory effectors, and antimicrobial proteins. In autoimmune diseases such as rheumatoid arthritis, systemic lupus erythematosus, inflammatory bowel disease, and psoriasis, IL-17 promotes inflammation, bone destruction, and the production of autoantibodies by inducing the expression of target genes. In contrast, IL-17 plays a crucial role in host defense against infections, particularly at mucosal sites, by enhancing the production of pro-inflammatory cytokines, chemokines, and antimicrobial peptides. The mechanisms of IL-17 signaling involve the activation of multiple signaling pathways, including NF-κB and C/EBP, and the cytokine's ability to synergize with other cytokines to enhance target gene expression. Understanding the complex roles of IL-17 in both protective and pathological processes is essential for developing targeted therapies.Interleukin-17 (IL-17) is a cytokine that plays a dual role in mammalian immunity, acting both as a host defender against infections and as a pro-inflammatory cytokine in autoimmune diseases. The induction of new gene expression by IL-17 is a key mechanism through which it exerts its effects. IL-17 targets a wide range of cells, including mesenchymal and epithelial cells, and induces the production of various cytokines, chemokines, inflammatory effectors, and antimicrobial proteins. In autoimmune diseases such as rheumatoid arthritis, systemic lupus erythematosus, inflammatory bowel disease, and psoriasis, IL-17 promotes inflammation, bone destruction, and the production of autoantibodies by inducing the expression of target genes. In contrast, IL-17 plays a crucial role in host defense against infections, particularly at mucosal sites, by enhancing the production of pro-inflammatory cytokines, chemokines, and antimicrobial peptides. The mechanisms of IL-17 signaling involve the activation of multiple signaling pathways, including NF-κB and C/EBP, and the cytokine's ability to synergize with other cytokines to enhance target gene expression. Understanding the complex roles of IL-17 in both protective and pathological processes is essential for developing targeted therapies.