B cells and follicular helper T cells express the receptor CXCR5 on their surface and utilize the chemokine CXCL13 for trafficking [22,23]; thus, intrathecal production of CXCL13 represents a critical contributory step for the development of relapses in MS and the formation of ectopic lymphoid follicles. This evidence concerns the gene CXCL13 and myeloid sarcoma.