"We have provided the first direct evidence that a T cell gives precise mechanical tugs to other cells," said Khalid Salaita from Emory University in the US.
"A tug that releases easily, similar to a casual handshake, signals a friend. A stronger grip indicates a foe," he added.
T cells continuously patrol through the body in search of foreign invaders. They have molecules known as T-cell receptors (TCR) that can recognise specific antigenic peptides on the surface of a pathogenic or cancerous cell.

When a T cell detects an antigen-presenting cell (APC), its TCR connects to a ligand, or binding molecule, of the APC, researchers said.
If the T cell determines the ligand is foreign, it becomes activated and starts pumping calcium. The calcium is part of a signalling chain that recruits other cells to come and help mount an immune response, they said.

Researchers hypothesised that mechanical strain might also play a role in a T cell response, since the T cell continues to move even as it locks into a bind with an antigenic ligand.

To test the idea, they developed DNA-based gold nanoparticle tension sensors that light up, or fluoresce, in response to a miniscule mechanical force of a piconewton - about one million-millionth the weight of an apple.

The researchers designed experiments using T cells from a mouse and allowed them to test ligands containing eight amino acid peptides that had slight mutations.

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