Human IL-17A (Interleukin-17A) is a pro-inflammatory cytokine belonging to the IL-17 family that plays a central role in host defense and immune-mediated inflammation. IL-17A is produced primarily by Th17 CD4⁺ T cells, as well as by γδ T cells, innate lymphoid cells (ILC3s), and other activated lymphocyte subsets in response to microbial infection and inflammatory stimuli. It acts on epithelial, endothelial, and stromal cells to induce the expression of pro-inflammatory cytokines (e.g., IL-6, TNF-α), chemokines (such as CXCL8/IL-8), and antimicrobial peptides, thereby promoting neutrophil recruitment and enhancing mucosal and barrier immunity. In healthy individuals, basal IL-17A levels are typically low, but concentrations increase during bacterial and fungal infections, particularly at mucosal surfaces. Dysregulated IL-17A signaling has been strongly implicated in chronic inflammatory and autoimmune diseases, including psoriasis, psoriatic arthritis, ankylosing spondylitis, rheumatoid arthritis, inflammatory bowel disease, and multiple sclerosis. Targeted inhibition of IL-17A (e.g., with monoclonal antibodies such as secukinumab and ixekizumab) has demonstrated significant clinical efficacy in several of these conditions. In clinical and translational research, human IL-17A serves as a key biomarker of Th17-mediated immune activation and neutrophilic inflammation and represents an important therapeutic target in immune-mediated inflammatory disease.