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It鈥檚 been right behind our noses all along but we鈥檝e only just discovered it. When the cells inside your nose sense danger, they release billions of tiny sacs filled with bacteria-killing weapons into the mucus lining.

These sacs not only kill bacteria directly, they also warn other cells of the danger and even help arm them against the invaders.聽This defence system has never been identified before.

鈥淲e have demonstrated in a live patient that the immune system reaches outside of the body, and actually goes and attacks pathogens before they get into the body,鈥� says Benjamin Bleier, a sinus surgeon at the teaching hospital Massachusetts Eye and Ear. 鈥淚t is the only example of this I know of.鈥�

The nose is a crucial frontier. Every breath we take may contain dangerous bacteria. So the cells lining our nasal cavity secrete a mucus that traps tiny particles. Hairs on the surface of these cells, called cilia, beat back and forth to move the mucus along.

What鈥檚 surprising, says Bleier, is that instead of being swept forwards into the nose so it could be rapidly expelled from the body, the mucus is swept backwards towards the throat. 鈥淵ou swallow it and then the gut deals with it from there,鈥� says Bleier.

As powerful as antibiotics

Bleier鈥檚 team and other researchers have recently found that, as well as secreting mucus, the cells of the nasal cavity also release billions of tiny sacs called聽.聽After being released into the mucus, these sacs can go on to fuse with other cells, delivering cargo such as proteins or RNA.

This聽made Bleier and his colleagues suspect that exosomes are part of a previously unknown defence system. Now, after studying tissue in the lab and people undergoing nasal surgery, the team have strong evidence for this idea.

They found that, within five minutes of the cells at the front of the nose being exposed to a potentially dangerous bacterium, the number of exosomes released into the mucus doubles. Their experiments suggest that these exosomes can kill pathogens directly, although we don鈥檛 yet know how. 鈥淭hey are as powerful at killing bacteria as an antibiotic,鈥� Bleier says.

Arm your neighbours

But not all exosomes kill. Many of the exosomes that don鈥檛 attack bacteria instead fuse with cells towards the back of the nose. In doing so, these exosomes seem to both alert these cells to the danger, and arm them with antibacterial proteins to fight them.

Bleier thinks this explains why the mucus is swept backwards. 鈥淢ucociliary clearance is not just a garbage dump,鈥� says Bleier. 鈥淚t鈥檚 actually a circulatory system.鈥�

The team now hope to identify聽how exosomes fuse to cells. This could allow us to develop artificial exosomes that could deliver drugs to the body more efficiently.

The findings make sense, says Cecilia Lasser of the University of Gothenburg in Sweden, whose team聽has also been examining whether exosomes may have a role in our immune system.

Exosomes were first discovered in 1983 but . As well as seeming to arm our noses against invaders, exosomes are also involved in all kinds of processes, from normal body functioning, to diseases including cancer and asthma.