2024-03-17 15:50:50
Why do more people get sick in winter? A study provides evidence of how cold and a weakened immune system are related.
Cough, runny nose, sore throat: colds are much more common in the cold season than in spring or summer. It is well known that the frequency of respiratory diseases is subject to seasonal fluctuations, but the underlying mechanisms are less so.
Whether cold can promote the development of colds has been a controversial theory in science. A study from the USA provides evidence that supports this assumption.
First contact via the nasal mucosa is crucial
The generic term cold usually refers to a respiratory tract infection caused by a virus. Up to 200 different viruses can cause a cold. The viruses are transmitted through droplet infection – that is, through sneezing and coughing – or through close contact with infected people.
Along with the mouth, the nose is the first entry point for viruses and bacteria. If the pathogens are inhaled or enter the nose directly – for example through the hands – they can adhere to the mucous membrane there. A number of defense mechanisms have developed in the nasal mucosa for protection. Cold alone cannot cause a cold. There is always a need for a pathogen. However, cold can play a role in whether an illness breaks out or not. What is crucial is their effect on the immune system.
The study from Harvard Medical School in Boston, published in the journal “Journal of Allergy and Clinical Immunology”, examined how the immune defense of the nasal mucosa is influenced by cold. The scientists focused on the so-called extracellular vesicles (EVs) – a part of the local immune system that is responsible for defending against bacteria and viruses in the nasal mucosa.
Extracellular vesicles (EVs)
EVs are small membrane vesicles formed by cells that they release into their environment. They are found in practically all body fluids and enable the exchange of information from cell to cell. EVs can contain components of genetic information, proteins or antibacterial and antiviral components.
Viruses trigger massive vesicle release
The scientists’ assumption was that cold hinders the process of vesicle formation. To find out, they first analyzed how the temperature in the nose changes when it is cold. The result: At winter temperatures of around 4 °C, the temperature in the nose drops from 37 °C to 32 °C within 15 minutes.
The researchers then examined how cells in the human nasal mucosa responded to three different viral infections: a coronavirus that causes the common cold and two different rhinoviruses. They analyzed vesicle formation at the reduced temperature of 32 °C and at a normal body temperature of 37 °C.
The result confirmed: Under normal conditions, an infestation with pathogens triggers the formation of a large number of extracellular vesicles in the nasal mucosa. The vesicles bind to the viruses, kill them and thus prevent direct contact with the nasal mucosa.
Cold impairs local immune defenses
However, in cold weather, the amount of EVs decreased by almost 42 percent, and the antiviral defense mechanisms of the EVs were also impaired. As a result, the immune system reacted weaker to viral infections. Because: The more EVs are formed, the lower the probability that the viruses can bind to the nasal mucosa.
The nasal EVs can therefore effectively suppress respiratory viral infections, but are impaired by cold stimuli. And: Mucous membranes have poorer blood circulation in the cold and are often drier, meaning they cannot perform their protective function as well. The study authors therefore consider it necessary to further investigate how the defense mechanisms in the nose could be supported at low temperatures.