By using full-length human Dsg3 or Dsg3-specific epitopes for ex vivo stimulation, we show that expression of the activating CD154 of PBMCs from PV patients can be found in CXCR5+ T follicular helper cells associated with IL-21 expression, further supporting the concept that antigen-specific T cells are central for the pathogenesis of PV. The gene discussed is CXCR5; the disease is acquired polycythemia vera.