IGHG3 and myalgic encephalomeyelitis/chronic fatigue syndrome: The five patients with ME/CFS showed average IgG3 levels at 1835 mg/L and average IgG4 levels at 881 mg/L, both of which can be considered as mild elevations, while the rest of the patients had IgG3 levels at 538 mg/L and IgG4 levels at 424 mg/L.