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

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

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

Antigen-presenting cells collect fragments of proteins from foreign bodies (normally pathogens, such as viruses and bacteria, but also transplanted organs) and show these to T cells, in order to tell them what to attack.

ECONOMIST: Breaching the body's defences