IL12B encodes the p40 subunit of interleukin-12 and interleukin-23 (IL-23), forming a heterodimeric cytokine critical for innate and adaptive immunity. IL12B associates with IL23A to create IL-23, which binds to IL12RB1/IL23R receptors and activates JAK-STAT signaling, preferentially stimulating memory T cells and promoting pro-inflammatory cytokine production 1. In tumors, IL-12 production by dendritic cells depends strictly on interferon-γ signaling and is negatively regulated by IL-4 1. IL-23 induces autoimmune inflammation and drives psoriasis pathogenesis, where IL4I1+CD200+CCR7+ dendritic cells are identified as primary IL-23 producers by co-expressing IL-23A and IL12B subunits 2. Therapeutically, ustekinumab—a monoclonal antibody blocking IL-12 and IL-23—significantly improves psoriasis and psoriatic arthritis outcomes 3. Genetically, IL12B polymorphism rs3212227 associates with susceptibility to type 1 diabetes, rheumatoid arthritis, and Behçet's disease, particularly in East Asians 4, while hypermethylation of IL12B CpG islands correlates with ankylosing spondylitis pathogenesis 5. Mendelian randomization analysis identifies IL12B as having causal evidence for inflammatory bowel disease and its subtypes 6, establishing IL12B as a central target for autoimmune and inflammatory disorders.