IL21 and myeloid sarcoma: Lately, a study showed that the percentage of circulating CD4+CXCR5+PD-1+ T cells and the level of serum IL-21 were higher not only in NMOSD patients than in MS patients and healthy controls, but also in relapsing NMOSD patients than in remitting NMOSD patients, suggesting that TFH cells and IL-21 were related to disease activity [83].