p19 subunit of interleukin-23 (IL-23), a heterodimeric cytokine composed of a unique p19 subunit and a shared p40 subunit (which it shares with IL-12). IL-23 is primarily produced by activated dendritic cells and macrophages. Its receptor complex, expressed on immune cells like T cells and innate lymphoid cells, consists of the IL-12Rβ1 subunit (shared with IL-12) and the IL-23R subunit. The IL-23/IL-23R signaling axis is a master regulator of the Th17 cell lineage. It promotes the expansion and survival of Th17 cells, which secrete pro-inflammatory cytokines such as IL-17A, IL-17F, IL-22, and GM-CSF. These cytokines are central drivers of chronic tissue inflammation and pathology in autoimmune diseases, particularly in the gut mucosa of IBD patients.