Cross immunity: Why previous colds protect against a severe course of corona

The corona virus family also includes various pathogens that cause harmless colds or diarrhea. People who have already come into contact with the virus family as part of a mild cold could react with a faster immune response to the dangerous SARS-CoV-2.

Even with a harmless  cold , the immune system  reacts  . Among other things, it forms T helper cells, which will recognize similar viruses in the human body more quickly in the future, including  SARS-CoV-2 . This memory can also be referred to as cross immunity.

Cross immunity responsible for rapid immune response

Cross-immunity would be a first indication of why adults are more likely to become seriously ill than children and adolescents. Researchers at the Berlin Charité and the Max Planck Institute for Molecular Genetics got to the bottom of the cause of cross-immunity: The spike protein was analyzed. This viral protein enables the virus to dock onto human cells.

A section was identified in the spike protein that can already be found in harmless corona viruses. This position cannot change in the event of a mutation, otherwise the virus would lose its ability to infect.
People who have already come into contact with corona viruses as part of a harmless cold have developed T cells in addition to B cells, which are responsible for the formation of antibodies.

T helper cells recognize foreign viruses and are responsible for a rapid immune response. These specific T helper cells are also known as CD4 cells.

CD4 cells were already present in the blood of 35 percent of healthy blood donors who had no antibodies against SARS-CoV-2 and had therefore not yet been confronted with COVID-19. The special T cells also react to harmless corona viruses, which are common cold pathogens, especially in children.

T cells: important role in COVID-19

In the study, around 800 people who had not yet been infected with the  corona virus  were regularly tested and examined. Overall, 17 out of 800 people contracted COVID-19. Of these 17 people, ten had already acquired reactive CD4 cells as part of a previous infection with cold viruses. The result:

  • The immune system of these patients reacted faster and more effectively to the infection: antibodies against the COVID-19 disease could be formed after just three to nine days. The CD4 cells play an important role here, as they recognize the virus early and thus quickly stimulate the formation of antibodies.
  • Previous illnesses with harmless corona viruses could therefore build up a corona memory and protect the body from severe courses: However, this is not a guarantee for a mild course. However, this could explain why people react differently to the corona infection.
  • While some people’s immune systems are already in alarm mode and producing antibodies, the virus unexpectedly “attacks” people without CD4 cells and thus damages the body: by the time the immune system reacts, severe symptoms have often already appeared.

Existing CD4 cells could also positively influence the immune response through the corona vaccination. While it usually takes two weeks for antibodies to develop, people with cross-immunity develop them in less than a week.

Further studies also confirm a possible cross-immunity, but assume that the immunity will be reduced after some time and could only temporarily protect against severe COVID-19 disease.

With increasing age, the ability for cross-immunity as well as the performance of the immune system as a whole decreases. This could explain why children and younger people can achieve faster and higher protection. This is one of the reasons why people with immunodeficiency or the elderly should take advantage of the booster vaccination.

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