The cell in question is called the B cell. It comes from your bone marrow, and its job is to create antibodies that latch onto infectious agents in your body, marking them for destruction by other components of the immune system.
In this study, researchers discovered that people with chronic fatigue syndrome had higher numbers of some types of B cells than healthy people in the control group.
While they don't know exactly what this means, researchers speculate that it could be an indicator of autoimmune activity.
Chronic fatigue syndrome has long been believed to involve an immune system "stuck in overdrive," but scientists have thus far been unable to determine whether autoimmunity is part of the excess activity.
In autoimmunity, the immune system mistakenly identifies a type of healthy issue as an infectious agent and works to destroy it.