.jpg) NK cell |
Null cells are large, circulating white blood cells that resemble T or B lymphocytes but which lack their characteristic surface receptors. The most common type of null cell is the natural killer cell (NK cell). A small minority of null cells are hematopoietic stem cells in free circulation around the body.[1] Aside from these normal human cells, mutant null cells may be associated with several forms of disease.
Pathological (abnormal and disease-associated) null cells have been found in some cancers, such as null cell adenomas of the pituitary gland. These tumors are usually slow-growing, but have poorly-understood patterns of hormone secretion and may cause necrosis of brain tissue in and around the tumor.[2] Null cells have also been identified in the nontumorous adenohypophysis, suggesting that null cell adenomas are derived from preexisting benign null cells.[3]
Cytomegalovirus is known to sometimes cause T-lymphocytes to stop expressing CD28 and other vital surface molecules, rendering them as a form of null cell. However, since they lack other features of NK cells, these altered lymphocytes do not contribute to the immune system and are thus associated with immunodeficiency.[4]