Why do more people get sick in the winter? A new study now provides evidence of how a cold and a weakened immune system are linked.
Cold and flu infections are more common in the cold season than in spring or summer. It is known that the frequency of respiratory disease is subject to seasonal fluctuations – but the underlying mechanisms are less so.
That cold can promote the development of the common cold was a controversial theory in science previously. A current study from the United States now provides new evidence to support this assumption.
The first contact is through the nasal mucosa
The generic term “cold” usually refers to a respiratory infection caused by viruses. Up to 200 different viruses can cause a cold. Viruses are transmitted through droplets – that is, by sneezing and coughing – or through close contact with infected people.
The nose is the first entry point for viruses and bacteria besides the mouth. If pathogens are inhaled or get directly into the nose – for example via the hands – they can attach themselves to the mucous membrane there. Therefore a number of defense mechanisms have been developed in the nasal mucosa for protection.
In the new study published in the “Journal of Allergy and Clinical Immunology” by Harvard Medical School in Boston, it was investigated how the immune defense of the nasal mucosa is affected by cold. The scientists turned their attention to the so-called extracellular vesicles (EVs) – part of the local immune system responsible for defending against bacteria and viruses in the nasal mucosa.
extracellular vesicles (EVs)
EVs are small sacs of membrane formed by cells that release them into their environment. They are found in almost all body fluids and enable the exchange of information from one cell to another. EVs can contain genetic information components, proteins, or antibacterial and antiviral components.
Viruses give rise to a huge vesicle release
Scientists hypothesized that the cold hinders the process of vesicle formation. To find out, they first analyzed how nasal temperature changes when it’s cold. The result: With winter temperatures of around 4°C, the temperature in the nose drops to 32°C in 15 minutes.
The researchers then looked at how human nasal mucosal cells respond to three different viral infections: the common cold coronavirus and two different nasal viruses. They analyzed vesicle formation at a reduced temperature of 32 °C and at a normal body temperature of 37 °C.
The result is confirmed: under normal conditions, infection with pathogens leads to the formation of a large number of extracellular vesicles in the nasal mucosa. The vesicles bind to viruses and kill them, thus preventing direct contact with the nasal mucosa.
Cold affects local immune defenses
However, when it was cold, the amount of EVs decreased by about 42 percent, and the anti-virus EVs defenses were also compromised. As a result, the immune system reacts weaker to viral infections. Because: the higher the formation of EVs, the lower the probability of viruses attaching to the nasal mucosa.
Thus nasal compounds can effectively suppress respiratory viral infections, but they are affected by cold triggers. The study authors therefore consider it necessary to further investigate how defense mechanisms in the nose are supported at lower temperatures.