Prof De Meirleir runs several tests on his patients and one of them includes CD56 cells. If I remember correctly a lower number of CD56 cells is connected to a weaker immune system (like in CFS). But I also found this study which comes to the conclusion that the lower the number of CD56 cells, the higher the symptoms of depression get.
Natural Killer T cells in patients with major depressive disorder.
Park EJ, Lee JH, Chae JH, Lee KH, Han SI, Jeon YW.
Department of Psychiatry, Our Lady of Mercy Hospital, The Catholic University of Korea, #665, Bupyeong 6-dong, Bupyeong-gu, Incheon, 403-720, Republic of Korea.
CD56 (Natural Killer T) cells showed a significant negative correlation with depressive symptom scale scores in acute and unmedicated patients with major depressive disorder. Decreased CD56 cells may reflect the severity of depressive symptoms but not the severity of anxiety symptoms in major depression.
PMID: 17027089 [PubMed - indexed for MEDLINE]