Given the well-equipped function as potential APCs, CD21low B cells might present potential targets for therapeutic interventions in AI patients, since the high expression of co-stimulatory molecules on these cells is combined with an increased incidence of auto-reactive BCR entities among CD21low B cells in RA, CVID, and SLE (12, 14, 17). Here, BCR is linked to systemic lupus erythematosus.