IL21 and Behcet disease: BD patients with active and untreated disease have high blood circulating levels of Th17 cells and low levels of Treg cells mediated by IL-21 which correlates with disease activity.27 IL-21 is produced by CD4+ T-cells.12 Activated Th17 cells produce IL-17 under the influence of IL-6 produced after antigenic (extracellular pathogens) activation of the innate immune system, which upregulates adhesion molecule expression on endothelial cells.28 Therefore, enhanced Th17 cellular activity plays a role in the vascular inflammation and thrombosis in BD.