IL21 and rheumatoid arthritis: IL‐21 is mainly produced by Tfh and Th17 cells, and this cytokine is known to regulate both germinal center (GC) B cell survival and plasma cell differentiation.[21] Importantly, these in vitro results showing salivaricin A2‐ and B‐mediated reductions in the IL‐21 level of PBMCs are consistent with the negative correlations we detected in our RA patient clinical data between the capacity of the tonsillar microbiome to produce salivaricins and the proportions of circulating precursor Tfh and plasma cells.