Lymphocytes Null cells

Null cells are also known as large granular lymphocytes , but are best known as natural killer (NK) cells. These represent a third lymphocyte subgroup. They are capable of directly lysing cancer cells and virally infected cells. 

Lymphocytes White blood cells formed in the body s lymphoid tissue. The nucleus is round or ovoid with coarse, irregularly clumped chromatin while the cytoplasm is typically pale blue with azurophilic (if any) granules. Most lymphocytes can be classified as either T or B (with subpopulations of each) those with characteristics of neither major class are called null cells. [NIH]... 

Lymphocytes are responsible for the specificity of the immune response. They are the only immune cells that recognize and respond to specific antigens, due to the presence on their surface of high-affinity receptors. In addition to blood, lymphocytes are present in high numbers in the spleen and thymus. They may be sub-categorized into antibody-producing B lymphocytes, T lymphocytes, which are involved in cell-mediated immunity, and null cells. 

Null cells are a separate subset of lymphocytes that lack surface markers of B or T origin. These cells, also referred to as large granular lymphocytes, are thought to perform functions such as direct cytotoxicity to foreign entities, and they act either alone (natural killer cells) or in concert with immunoglobulin (antibody-dependent cellular cytotoxicity). (Further details regarding lymphocytes are found in Chap. 84, on the immune system.)... 

Recently few interesting observations show that clonality analyses may be also useful to analyze some types of classical Hodgkin s lymphoma, namely nodular sclerosis and lymphocyte predominance type, as the vast majority of the neoplastic L H cells (about 95%) proved to be of B-cell origin, i.e. from germinal center B-lymphocytes. The rest ( 5%) of L H cells seems to be of T- or null-cell origin. The IgH rearrangement is found in up to 20% of Hodgkin s lymphoma types. 

Cowan, M., Fraga, M., Andrew, J., Lameris-Martin, N., and Ammann, A., 1982, Purine salvage pathway enzyme activities in h iman T, B and null lymphocyte populations, Cell. Immunol., 67 121. 

Natural killer (NK) cells are a subpopulation of lymphocytes that can lyse a wide variety of tumor cells and some normal cells. They are classified as null cells, being neither B or T cells. The assay usually is an in vitro one in which target cells, labeled with 51Cr are exposed to the NK cell population, and the amount of radioactive 51Cr released is measured. NK cell activity is highest in young animals (mice 5-8 weeks of age), and is present in very low levels, if at all, in older animals. However, when older rats were exposed to C. parvum or BCG, there was an increase in the cytotoxicity of the lymphocyte population. 

Interferona (IFNa) is the major interferon produced by virus-induced leukocyte cultures the primary producer cells are null lymphocytes, and the major activities are antiviral activity and activation of NK cells. It is used in the experimental treatment of hairy cell leukemia and other selected neoplasias. 

Recently, a carbohydrate-binding protein, L-selectin, has been proposed as an adhesion molecule for human embryo implantation (19). L-Selectin is expressed on the surface of lymphocytes and interacts with sulfated and fucosylated carbohydrates expressed on lymph node endothelial cells (20-26). Carbohydrate ligand for L-selectin in the lymph node is closely related to MECA-79 antigen (Fig. 1). In human endometrium, the expression of MECA-79 antigen is hormonal cycle dependent but the presence of an embryo is not required (27). In the mouse, L-selectin nulls reproduce normally (28). Mutant mice lacking sulfotransferases (22,23,29-32) and glycosyltransferases (21,31,32) required for synthesis of L-selectin ligand exhibited no defects in reproduction. Therefore, it is clear that L-selectin is not required for the mouse embryo implantation. It thus appears that L-selectin is uniquely involved in human embryo implantation. 

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