IL12A encodes interleukin-12 subunit A, a critical cytokine in immune regulation. IL12A heterodimerizes with IL12B to form IL-12, produced by professional antigen-presenting cells including dendritic cells, B cells, and macrophages 1. IL-12 regulates T-cell and natural killer-cell responses, induces interferon-gamma production, and promotes T-helper 1 (Th1) cell differentiation, linking innate and adaptive immunity 2. Mechanistically, IL-12 signals through heteromeric receptors (IL12R1/IL12R2) to activate JAK kinases and phosphorylate STAT4, which translocates to the nucleus to regulate gene expression 34. IL12A also heterodimerizes with EBI3 to form IL-35, which maintains immune homeostasis and suppresses immune responses through STAT1/STAT4-dependent signaling 5. IL12A variants are associated with multiple diseases. Genetic polymorphisms increase susceptibility to celiac disease, multiple sclerosis, and Behçet's disease 678. IL12A variants also predispose to clonal hematopoiesis with aging 9. Clinically, IL12A upregulation in dendritic cells enhances CD8+ T-cell anti-tumor immunity in colorectal cancer, and IL-12 production promotes anti-tumor FOXP3-low T cells 1011. During COVID-19, IL12A expression is reduced in association with dysbiosis 12.