INTRODUCTION Natural killer (NK) cells are non-T, non-B lymphocytes that are critical in defense against virally-infected cells and in tumor surveillance [1-3]. These cells are considered part of the innate immune system because they do not require previous exposure to foreign, pathogenic, or dangerous antigens to act. The importance of NK cells in human health and disease is illustrated by a small number of human diseases in which NK cells are absent or defective. These conditions are characterized primarily by severe, recurrent, or atypical infections with herpes viruses. The biology of NK cells, the clinical manifestations of isolated NK cell deficiency syndromes, the evaluation of patients suspected to have these disorders, and the differential diagnosis will be discussed here. The management of patients with NK cell deficiency syndromes is presented separately BIOLOGY OF NK CELLS NK cells are lymphocytes that originate in the bone marrow that comprise 5 to 10 percent of the peripheral blood lymphocyte pool . Although low in number, they are the main lymphocytes of the innate immune system in the peripheral blood. NK cells account for a minority of lymphocytes within organs in most cases, although there are important exceptions. INTRODUCTION NK cell deficiency syndromes are rare disorders in which NK cells are absent, deficient, or dysfunctional, in the absence of any other identifiable immunodeficiency, genetic disorder, or medication known to affect NK cells. The management of patients with these disorders is discussed here. The biology of NK cells, the clinical manifestations of NK cell disorders, and the evaluation of patients suspected of having NK cell defects are presented separately. (See "NK cell deficiency syndromes: Clinical manifestations and diagnosis".) OVERVIEW Therapy for patients with NK cell deficiency syndromes is largely empiric, due to the small numbers of cases described in the literature. Active infections must be identified and treated aggressively, as with any immune disorder. The use of specific therapies is based either on theoretical utility or anecdotal reports of benefit (table 1). ANTIVIRAL PROPHYLAXIS NK cell disorders are characterized clinically by susceptibility to severe and/or recurrent infection with herpes viruses, including varicella zoster virus (VZV), Herpes simplex virus (HSV) I and II, Epstein Barr virus (EBV), and cytomegalovirus (CMV). There is also a marked susceptibility in some patients to Human Papilloma Viruses. Prophylactic antiviral regimens should be tailored to the infectious history of the individual patient. Serologic tests should be performed to determine if an NK cell deficient individual has experienced infections with known herpesviruses. In our clinic, we assess for past exposure to the following viruses: http://www.uptodate.com/contents/nk-cell-deficiency-syndromes-clinical-manifestations-and-diagnosis http://www.uptodate.com/contents/nk-cell-deficiency-syndromes-treatment?source=see_link Is this what cfs/me is?? cheers!!