Flow cytometric analysis of human naive CD4+ T cells stimulated for 3 days with anti-CD3/CD28–coated beads alone or in the presence of ME/CFS patients’ sera (2.5% vol/vol), with or without IL-12, showed that all serum samples tested (n = 24) induced the expression of the TFH cell signature markers programmed cell death 1 (PD-1) and CXCR5 (Figure 2A, bottom row representative plots, and Figure 2B) (up to 38.9% just by serum alone) compared with cells stimulated with anti-CD3/CD28–coated beads plus control serum (Figure 2A, representative top row plots, and Figure 2B). Here, CXCR5 is linked to myalgic encephalomeyelitis/chronic fatigue syndrome.