TFRC and juvenile idiopathic arthritis: Indeed, B-cell cluster 18, which was found almost exclusively in the joint (47.3 ± 5.5% of SF B cells vs 3.1 ± 1.4% of HC PB and 2.7 ± 0.3% of JIA PB B cells, Fig. 3D), were characterised by high CD71 expression (Fig. 3B–D), a transferrin receptor upregulated on activated lymphocytes.