TNFSF13B and Autoimmunity: BLyS is expressed by activated T cells, activated macrophages, and dendritic cells [1,3,4] and has been implicated in autoimmune disorders characterized by the presence of pathological concentrations of self-antigen-reactive antibodies, such as systemic lupus erythematosus (SLE) [5] and rheumatoid arthritis (RA) [6].