IL-17A, also known as CTLA-8, is a significant member of the IL-17 cytokine family. It is primarily secreted by Th17 cells, but is also produced by various immune cells including CD8+ T cells, γδT cells, NKT cells, monocytes, and neutrophils. IL-17A plays a crucial role in host defense against a variety of bacterial and fungal pathogens and is also involved in allergic and autoimmune responses. It induces the production of antimicrobial peptides, cytokines, chemokines, and matrix metalloproteinases, promoting neutrophilic inflammation and playing a role in antiviral responses. IL-17A is a homodimeric cytokine that binds with high affinity to IL-17RA, which is necessary for the biological activity of IL-17A. IL-17A is associated with multiple autoimmune diseases such as rheumatoid arthritis, multiple sclerosis, inflammatory bowel disease, asthma, and psoriasis. In cancer, IL-17A also plays a pathogenic role by recruiting myeloid-derived suppressor cells (MDSCs) to suppress antitumor immunity and promoting tumor growth in vivo through the induction of IL-6. Therefore, IL-17A is an important target in the treatment of related diseases, as it plays a significant role in inflammation, immune responses, and tumor development.