Gastrointestinal system

Dysbiosis
ME/CFS patients have lower levels of Bifidobacteria and higher levels of aerobic bacteria.

Intestinal permeability
One study found higher levels of serum [IgA|Immunoglobulin A] and IgM against lipopolysaccarides (LPS), a major component of the outer membrane of gram negative bacteria. . Serum IgA was significantly correlated to the severity of illness.