A particular strain of hairless mice genetically lack a thymus and T lymphocytes, yet these mice do not appear to have an especially high incidence of tumor production. This surprising observation led to the discovery of natural killer (NK) cells, which are lymphocytes that are related to, but distinct from, T lymphocytes. Unlike killer T cells, NK cells destroy tumors in a nonspecific fashion and do not require prior exposure for sensitization to the tumor antigens. The NK cells thus provide a first line of cell-mediated defense, which is subsequently backed up by a specific response mediated by killer T cells. These two cell types interact, however; the activity of NK cells is stimulated by interferon, released as one of the lymphokines from T lymphocytes.
Recent evidence suggests that NK cells particularly attack cells that lack class-1 MHC antigens. As previously mentioned, all of a person's normal tissue cells display this antigen. The method of attack is similar to that of the killer (cytotoxic) T lymphocytes: they release perforin proteins and the granzyme enzyme. Perforins insert into the victim plasma membrane, polymerize, and thereby form a large pore in the membrane. Granzyme is taken into the victim cell and indirectly leads to the destruction of its DNA.
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