The researchers saw signs that the immune system had become “worn out” in ME/CFS participants. In the blood of this group, they found that killer T cells, which normally target infected cells for destruction, had reduced levels of a protein called CD226 that would otherwise boost their proliferation and activity. In the cerebrospinal fluid, killer T cells had elevated levels of programmed cell death protein 1 (PD1).

This protein is considered an “exhaustion” marker, and its presence can indicate that an overstimulated T cell has shut down. “The immune system in effect burns out, becomes exhausted and can no longer respond to infectious triggers.”