They include compounds that block bad genes responsible for spreading cancer into the bones, chemicals that prompt prostate cancer cells to destroy themselves and vaccines that train the body's killer T-cells to seek and destroy tumor cells.

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Two specific types of immune system cell that attack invaders - CD4 T-cells and natural killer cells - increased in number with age, with a higher rate of increase in women than in men.

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In addition, other types of cancer can interfere with the activation of "killer T cells" - immune cells which arrive to destroy tumour cells.

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Pfizer's drug tremelimumab and Bristol-Myers Squibb's ipilimumab are antibodies to a protein called CTLA-4 (cytotoxic T-lymphocyte antigen-4) that acts as an emergency brake to prevent killer T cells from attacking healthy tissue.

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