Scientists believe they can explain why some people who become infected with Covid develop some abnormalities in the skin, on the part of the fingers.
It has been described as the 'Covid Finger', which is another side effect, which according to experts occurs in rare cases.
The researchers say they have identified parts of the immune system that appear to be involved.
The findings, in the British Journal of Dermatology, may help with treatments to relieve symptoms.
What is the 'Covid Finger'?
It can occur at any age, but affects children and adolescents more often.
For some it is painless, but sometimes skin rash can be extremely painful and cause itching as well as blisters or swelling.
Why does it happen?
These latest study findings, based on blood and skin tests, suggest that two parts of the immune system may be at play.
Both involve mechanisms that the body uses to fight the coronavirus.
One is an antiviral protein called interferon type 1, and the other is a type of antibody that mistakenly attacks a person's own cells and tissues, not just the invading virus.
Researchers hope the findings will help patients and doctors better understand the condition.
Dr. Veronique Bataille, a consulting dermatologist and spokeswoman for the British Skin Foundation, said this symptom appeared very often during the early stages of the pandemic, but has been less common in the current wave of the Delta variant.
This may be due to more people being vaccinated or protection against Covid from past infections.
