Natural killer (NK) cells are interesting little beasties. They represent part of the innate immune system that is able to kill without the normal MHC dependent recognition system. They have resposibilities as effector cells to make antibodies kill, but also are used by the body to kill virus infected cells while the T cells are reving up the specific response. They are activated by gamma interferon and other macrophage induced cytokines.
NK cell tumors are very rare, especially among Caucasians.
There exists an NK cell lymphoma of the nasal cavity which occurs mainly in Orientals, Asians and South Americans and is EBV related. When it occurs in the blood it is rapidly progressive and drug resistant.
There is also an NK cell leukaemia, again commoner in Asians than Caucasians that presents as a large granular lymphocytosis. Again this may be associated with EB virus, and again it is rapidly progressive and difficult to treat. It occurs in teenagers and young adults.
There is a chronic indolent large granular lymphocytic leukaemia, the majority of which are T-cell type. It tends to be associated with splenomegaly, neutropenia, anaemia and arthritis. Perhaps 10% of them are of NK cell type.
To distinguish between the T cell type and the NK cell types we must do lymphocyte markers. The NK cell type is CD2+, CD7+, CD8+, CD56+, CD16+ but CD3-, CD5-, CD57-.
The NK cell type is less likely to have anemia, neutropenia, splenomegaly and arthritis, however some cases have been associated with a cutaneous vasculitis, usually present before the LGL leukemia appears.
Often treatment is unnecessary, but when it is ciclosporin should be first line, rather than chemotherapy.