Staying out of the sun could be harmful for your organism.

That's because sun tanning is not only good for your physical appearance, but also for your health.

A recent study found that sunlight enhances the immune system against invading germs and sun-induced skin damage. The T cells of the immune system are responsible for fighting against infections and cancer, but their activity is triggered by information about the threat. These are brought by the dendritic cells, which ingest infected and damaged cells and transport the regurgitated pieces to T cells.

If the T cells perceive foreign elements, they multiply and the huge number of T cells destroys infected cells. But scientists were puzzled how T cells could receive information from all around the body.

Previous research on the gut found that dendritic cells expel a compound that induces T cells to generate a receptor that helps them head for the intestine.

A team led by immunologist Eugene Butcher at Stanford University, California, found a similar immune process in human skin.

Under the action of sunlight, skin cells produce an inert form of vitamin D, which was long thought to become active and thus usable by the body after processed by the kidney and liver. But the team found that dendritic cells in the skin can do it. The "active" vitamin D3 reaches nearby T cells, inducing them to generate receptors specific to skin chemokines.

With better sensors for these compounds, the alerted T cells create a beeline for the uppermost skin layer, where they began destroying infected cells. "The findings explain how T-cells "know" to go to the skin's surface once the skin has suffered some sun-induced DNA-damage," said the researchers. "Getting some sun is good for building the skin's defenses" said Butcher. "Sunshine is good for you, as long as it's not too much," says team member Hekla Sigmundsdottir.

"The skin disorder psoriasis is sometimes treated with vitamin D3 creams - it may work by moving T-cells into the skin," she speculates.

"It doesn't take much sun to get the vitamin D the body needs, however, and too much sun still poses a dangerous risk of skin cancer," the authors note.

"It's work of extremely high standards," says vitamin D expert Chantal Mathieu of the Catholic University of Leuven in Belgium. "Hints of the story had been known from previous studies, but no one had made sense of it until now," she said.