Not only does tobacco promote cancer development, but it also helps early cancer cells avoid detection by the immune system. Usually, the immune system is able to mop up harmful cells before they multiply and spread.

Dr Jane McCutcheon and colleagues from New York University discovered that when cells are exposed to tobacco they have fewer substances on their surfaces which signal whether they are dangerous or not. This means that cancerous cells with less warning devices are more able to avoid detection by the immune system and be destroyed, but instead continue to spread and grow. The warning devices are called HLA class 1 molecules, and are usually unique to each person. HLA class 1 molecules sit on the surface of cells and present a protein to the outside environment. If the protein comes from harmless substances inside the cell, immune system components called killer T cells leave the cell alone, as it isn't a threat to the body. If the protein inside a class 1 molecule is a result of cancer in the cell, or a virus, then the killer T cells destroy it. Following experiments, Dr McCutcheon and colleagues found that cells exposed to tobacco had less HLA class 1 molecules. Another finding was that cells exposed to tobacco showed lower levels of a protein called TAP1, which forms a link in the chain that assembles HLA 1 class molecules in the cell. Reductions of TAP1 are likely to lead to less HLA class 1 on the surface of the cell, according to the researchers. Dr McCutcheon explained that these changes do not cause cancer, but allow the cancer to thrive in the body once it appears. She added that she was still unsure as to which specific ingredient in tobacco interfered with the production of HLA class1, but she said that any substance containing tobacco (this includes tobacco products that are not inhaled such as chewing tobacco) would have the same effect.

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