T-betCD21 B cells: the need to unify our understanding of a distinct B cell population in health and disease.

After many years of a niche research in a few laboratories of the world, T-betCD21 B cells have entered the limelight during the last years after the discovery of T-bet as common transcription factor of this unconventional B cell population and the increasing awareness of the expansion of these cells in autoimmune and infectious diseases. This population consists of different subsets which share large parts of their transcriptome, essential phenotypic markers, and reduced B cell receptor (BCR) signaling capacity. Inborn errors of immunity have helped to delineate essential signals for their differentiation. While our comprehension of their origin has improved, future research will hopefully profit from a common definition of the different T-betCD21 subpopulations in order to better define their specific roles during normal and aberrant immune responses.