In RRMS and high‐risk CIS blood, transitional (IgM+CD27−CD38hiCD24hi) as well as naive mature (IgM+CD27−CD38−/dim) B‐cell subsets displayed the highest CXCR4/CD74 expression ratios as compared to class‐switched (CD27+/CD27− IgG+ and IgA+) and nonclass‐switched (IgM+CD27+) memory subsets (Fig. 3; Supporting Information Fig. 2), implying that the CXCR4hiCD74lo phenotype of B cells in early MS reflects a more immature state. Here, CD74 is linked to myeloid sarcoma.