TNFSF13B and juvenile idiopathic arthritis: In addition, increased levels of cytokines relevant for B cell activation, maturation, differentiation and survival such as B cell activating factor (BAFF), A proliferation-inducing ligand (APRIL), IL-6 or IL-21, have been detected in serum and/ or synovial fluid from JIA patients (76, 89, 196–201).