Although reduced numbers of unswitched mB cells in patients with RA have been reported in 2009 and 2018 by using specific staining (e.g., CD19, CD27, and IgD) on isolated PBMCs or whole blood by using FACS or flow cytometry, respectively, we show for the first time that a reduced frequency of unswitched mB cells is the only significant difference distinguishing the immunophenotype from patients with RA from HC in a complex immunophenotyping panel of 72 different cell populations [10, 31]. Here, CD27 is linked to rheumatoid arthritis.