The cold wave has Germany firmly in its grip. A winter classic, we know that. But why is that? Researchers have now found a scientific explanation for the first time.

Runny nose, cough, sore throat – respiratory infections such as the flu are particularly widespread in winter. So far, it has been assumed that this is mainly due to the fact that many people are indoors and the viruses spread more easily there. An experimental study is now providing evidence for the first time of another, biological cause.

The research team led by Benjamin S. Bleier, Mansoor Amiji and Di Huang has therefore discovered a previously unknown immune reaction in the nose and found out that it is inhibited by cold. So-called extracellular vesicles (EV for short) play a key role here. The results were published last Tuesday in The Journal of Allergy and Clinical Immunology (JACI).

The nose is considered the gateway for numerous pathogens. As early as 2018, scientists, also under the direction of Bleier and Amiji, were able to demonstrate a previously unknown immune reaction there: cells in the front part of the nose recognized the bacteria and immediately began producing billions of simple copies of themselves, the so-called extracellular vesicles (EVs for short). ). The EVs are expelled from the cells into the nasal mucus, where they stop invading germs. The study found that the nose can increase the production of extracellular vesicles by 160 percent when attacked.

In a press release, Bleier compares it to “stabbing a wasp’s nest”. And explains: “While EVs can’t divide like cells, they’re like little mini versions of cells specially designed to kill these viruses.” EVs act as a kind of decoy – “so if you breathe in a virus, it sticks to those lures instead of sticking to the cells.”

In the new study from December 2022, the scientists led by Di Huang now examined whether the same applies to viruses in addition to bacteria. Viruses are known to be the trigger for most respiratory diseases such as corona, flu or a cold. Specifically, investigations were carried out with rhinoviruses (the most common cause of colds) and the corona virus. And indeed, all viruses triggered an EV swarm response of the nasal cells.

At normal temperatures, our nose is pretty well armed against viruses and bacteria. If it gets significantly cooler, however, things will look different.

The researchers simulated this as follows: Four healthy volunteers were exposed to a temperature of four and a half degrees Celsius for 15 minutes. The result: the temperature inside the nose dropped by around five degrees. And the amount of EVs secreted by the nasal cells decreased by almost 42 percent. The immune system’s ability to fight off respiratory infections is reduced by almost half.

The research provides “a logical explanation for the seasonal variation in upper respiratory tract infections,” Huang said. The researchers write that further studies should follow in the future. New drugs based on the study results are also conceivable.