The two most significantly altered proteins in PBMCs from ME/CFS patients are related to the immune system, cathepsin W (CTSW) (FC = 1.53), involved in the major histocompatibility complex (MHC) class I pathway, and human leukocyte antigen (HLA) protein histocompatibility antigen, alpha chain E, (HLA-E) (FC = 1.34) belonging to the MHC class Ib. Here, HLA-C is linked to myalgic encephalomeyelitis/chronic fatigue syndrome.