Some lymphocytes do not have characteristics of T or B cells, and although only malce up a small proportion of cells, may play an important role in immunity.
Natural killer (NK) cells
The role of NK cells is to eliminate tumour and virusinfected cells. This process is not antigen-specific. Many of the cells appear as large granular lymphocytes with an indented nucleus. The granules, which contain acid hydrolases including acid phosphatase, a-naphthyl acetate esterase and f3-glucuronidase, are thought to be involved in the cytotoxic events. NK cells are non-phagocytic, and most are CD4-, CD8- and surface immunoglobulin negative but carry the CD56+ CD2+ marker on their surface.
Antibody-dependent cytotoxic cells and Iymphokine-activated killer (LAK) cells. These are populations of lymphocytes that are not characterized by their surface molecules, but by function.
ADCC bear Fe receptors on their surface, and recognize target cells coated with immunoglobulin. They may have a role in eradicating virally infected cells and tumour cells. Incubation of lymphocytes with interleulcin-2 (IL-2) causes them to become highly cytotoxic (hence ‘lymphokine activated’), particularly to tumour cells. These have been used in the treatment of malignancies, where patients’ blood lymphocytes have been harvested, cultured with 1L-2 and then rein fused to target tumours.