The study has found a surprise benefit of sunshine - by providing an energy boost for infection fighting T cells, making them move faster.
Scientists already knew that sunlight allows us to make vitamin D, credited with healthier living.
But a surprise research finding could reveal another powerful benefit of getting some sun.
Researchers have found that sunlight, through a mechanism separate than vitamin D production, energises T cells that play a central role in human immunity.
Their findings, published in the journal Scientific Reports, suggest how the skin, the body’s largest organ, stays alert to the many microbes that can nest there.
Study senior investigator Doctor Gerard Ahern, an associate professor at Georgetown University Medical Centre in the US, said: “We all know sunlight provides vitamin D, which is suggested to have an impact on immunity, among other things.
“But what we found is a completely separate role of sunlight on immunity.
“Some of the roles attributed to vitamin D on immunity may be due to this new mechanism.”
The researchers specifically found that low levels of blue light, found in sun rays, makes T cells move faster - marking the first reported human cell responding to sunlight by speeding its pace.
Dr Ahern said: “T cells, whether they are helper or killer, need to move to do their work, which is to get to the site of an infection and orchestrate a response.
“This study shows that sunlight directly activates key immune cells by increasing their movement.”
Dr Ahern also added that while production of vitamin D required UV light, which can promote skin cancer and melanoma, blue light from the sun, as well as from special lamps, is safer.
And while the human and T cells they studied in the laboratory were not specifically skin T cells - they were isolated from mouse cell culture and from human blood - the skin has a large share of T cells in humans, according to Dr Ahern, about twice the number circulating in the blood.
He said: “We know that blue light can reach the dermis, the second layer of the skin, and that those T cells can move throughout the body.”
The researchers further decoded how blue light makes T cells move more by tracing the molecular pathway activated by the light.
What drove the response in T cells was synthesis of hydrogen peroxide, which then activated a signaling pathway that increases T cell movement.
Hydrogen peroxide is a compound that white blood cells release when they sense an infection in order to kill bacteria and to “call” T cells and other immune cells to mount an immune response.
Dr Ahern added: “We found that sunlight makes hydrogen peroxide in T cells, which makes the cells move.
“And we know that an immune response also uses hydrogen peroxide to make T cells move to the damage. This all fits together.”
He said there is further work to do to understand the impact of the findings, but he suggests that if blue light T cell activation has only beneficial responses, it might make sense to offer patients blue light therapy to boost their immunity.