Key differences in the immune responses to Covid-19 is one of the reasons why some people get sick while others fight off infection without knowing they have it, according to scientists.
UK researchers have found those who remain asymptomatic after catching coronavirus often have raised levels of a protective type of immune cell known as B cells, which is missing in people with severe symptoms.
The team also discovered that patients with mild to moderate symptoms have high levels of helper T-cells, which help fight infection, while those with serious symptoms have lost many of these immune cells.
They said the findings, published in the journal Nature Medicine, may in future help identify those who are more likely to develop severe disease by looking at levels of these immune cells in their blood and identifying potential treatments for them.
Professor Muzlifah Haniffa, senior author from Newcastle University and senior clinical fellow at the Wellcome Sanger Institute, said: “This is one of the only studies of its kind that looks at samples collected from asymptomatic people, which helps us start to understand why some people react differently to Covid-19 infection.
“It could also explain symptoms such as lung inflammation and blood clots.
“The immune system is made up of lots of different groups of cells, similar to the way an orchestra is made up of different groups of instruments, and in order to understand the coordinated immune response, you have to look at these immune cells together.”
The research, which involved experts from the Wellcome Sanger Institute, Newcastle University, University College London, University of Cambridge and EMBL’s European Bioinformatics Institute (EMBL-EBI), is part of the Human Cell Atlas project – an initiative to map every cell type in the human body to increase understanding of health, infection and disease.
The team analysed blood samples from 130 people with Covid-19 from UK centres in Newcastle, Cambridge and London, ranging from asymptomatic to critically severe.
They sequenced more than 800,000 individual immune cells, and found increased levels of B cells among those who were asymptomatic.
The researchers said these B cells produce antibodies that are found in mucus passages, such as the nose, and may be one of the first lines of defence against Covid-19.
These protective B cells were missing in people with serious symptoms, indicating the importance of an effective antibody-associated immune response at the nose and other mucus passages, the team added.
In contrast, the researchers said, people with severe disease had an uncontrolled increase in a type of white blood cell known as monocytes as well as killer T-cells.
While monocytes and killer T-cells help fight infection, high levels can lead to inflammation in the lungs.
Those who were critically ill were also found to have raised levels of cells that produce platelets, which help blood to clot.
The scientists said it is not yet understood how the infection stimulates these responses, but added their work offers an explanation as to how Covid-19 could cause an increased risk of blood clotting and inflammation in the lungs, which can lead to the patient needing a ventilator.