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. (See "NK cell deficiency syndromes: Treatment".)
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. In the duodenal epithelia and in the uterine decidua during pregnancy, NK cells account for up to 45 and 70 percent of lymphocytes, respectively [5,6].
Phenotypic characteristics — NK cells differ phenotypically from other lymphocytes by the presence of large cytoplasmic granules (picture 1).