IL12A encodes interleukin-12 alpha, a cytokine subunit that heterodimerizes with IL12B to form the IL-12 signaling complex or with EBI3 to form IL-35. IL-12 is primarily produced by professional antigen-presenting cells including dendritic cells, macrophages, and B cells, and regulates adaptive immunity by promoting interferon-gamma production and Th1 cell differentiation. Mechanistically, IL-12 signals through a receptor composed of IL12R1 and IL12R2 subunits, triggering JAK2 and TYK2 tyrosine kinase phosphorylation and subsequent STAT4 activation. As part of IL-35, IL12A also maintains immune homeostasis through unconventional receptors and functions as an immunosuppressive cytokine. Genetic variants in IL12A are reproducibly associated with multiple autoimmune and inflammatory conditions. Genome-wide association studies have linked IL12A polymorphisms to primary biliary cirrhosis, with a five-allele haplotype in the 3' flank conferring substantial disease risk, as well as to celiac disease, multiple sclerosis, and Behçet's disease. In colorectal cancer, IL-12 secretion by tissue-resident cells promotes the development of inflammatory FOXP3-negative T cells associated with improved prognosis, while probiotics enhance IL12A production in dendritic cells to prime CD8+ T cell immunity. Clinical interest in IL12A is reflected in approved therapies targeting the IL-12 pathway, including ustekinumab and briakinumab, which inhibit IL-12 and IL-23 signaling in inflammatory and autoimmune diseases.