CD4 and acquired polycythemia vera: Early indications of a role for Tfh cells in pemphigus came from studies showing that the frequency of circulating Tfh cells (defined as CD4+CXCR5+ T cells) and plasma concentrations of IL-21 were both significantly higher in patients with PV compared to controls (Holstein et al., 2021); and identified autoreactive IL-21-secreting cells in 50% of patients with PV (Hennerici et al., 2016).