When scientists realized that the immune system could discriminate self from non-self, they began to study whether the body could recognize tumor cells as foreign and then eliminate them. An immune response against foreign antigens (for example, viruses or bacteria) typically requires immunization and also that the foreign antigen binds to the body's own major-histocompatibiity [MHC] antigens (for example, HLA or H-2).

Using in vitro assays to measure the killing of tumor cells, researchers found that certain lymphocytes (a type of white blood cell responsible for recognizing foreign antigens) could lyse, or destroy, tumor cell targets. Surprisingly, these unique lymphocytes are present in the blood of normal, unimmunized individuals and they are able to recognize tumor cells that do not carry self MHC markers. The lack of requirement for immunization, and the ability to recognize tumor cells without MHC antigens, provided a working definition for a new type of cytotoxic,...

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