CXCL13 and rheumatoid arthritis: Although serum CXCL13 may originate from a variety of different sources, including lymph nodes where CXCL13 is constitutively produced [11,12] and which can exhibit aspects of reactivity in association with active RA [29], the inflamed synovial tissue appears to significantly contribute to the systemic up-regulation of the chemokine [15].