IL21 and immune system disorder: In this study we continued to investigate the mechanism(s) by which EBV and HHV-6A dUTPase proteins contribute to the immune dysfunction observed in ME/CFS and identified activin A, an early regulator of human follicular helper T cell (TFH) differentiation, as well as IL-21, a regulator of germinal center (GC) TFH cell survival and plasma cell differentiation, to be markedly increased in ME/CFS sera.