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

So it ought to be possible to test for the presence of a certain antigen on a surface by dragging a suitable antibody over it, and seeing if it gets stuck going fishing, in effect, on a molecular scale.

ECONOMIST: Microscopy

As the tip continued its movement across the surface, the antibody was snapped off the antigen and dragged along until it hit another receptive site.

ECONOMIST: Microscopy