Natural killer cells and immunity against cancer.

Extract: Under specific conditions, the immune response can eradicate tumors. The specific response against cancer requires the recognition of target molecules (antigens) by a subpopulation of white blood cells, the T lymphocytes. These antigens are exclusively or predominantly expressed by the tumor cells. Since many tumors fail to express specific antigens or express these antigens in a "wrong" way that impairs their recognition, cancer cells frequently escape from the specific immune response. Nonetheless, alternate effector cell populations have been described that are able to destroy tumor cells without any requirement for recognition of cancer-specific antigens. Among these effector populations, natural killer (NK) cells have an important role. Instead of looking for the presence of tumor cell-specific antigens, the NK cells sense the absence of molecules usually present at the surface of normal cells (the so-called human leukocyte antigen (HLA) class I molecules). Very often, these HLA class I molecules are not expressed, or are expressed but in an abnormal way by cancer cells. This "negative" mechanism of tumor cell recognition and destruction is called the "missing self hypothesis" since it postulates that the absence of molecules that identify the cell as "self" induces the destruction of these cells.