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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Both drugs turn off a natural brake on immune system activity called CTLA4 (cytotoxic T-lymphocyte antigen 4).

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However, before the alarm can be sounded in this way, proteins on the surface of the antigen-presenting cell and the T cell must bind together in what is called a cellular handshake.

ECONOMIST: Breaching the body's defences

To do this, they used antibodies designed to bind to and thus decommission the molecules on T cells that are used to shake hands with antigen-presenting cells.

ECONOMIST: Breaching the body's defences

For a T cell to become active, it must receive signals from another part of the immune system called an antigen-presenting cell.

ECONOMIST: Breaching the body's defences