By Richard Alleyne | Telegraph | Mar. 7, 2010

The so-called sunshine vitamin, which can be obtained from food or manufactured by human skin exposed to the sun, plays a key role in boosting the immune system, researchers believe.

In particular it triggers and arms the body's T cells, the cells in the body that seek out and destroy any invading bacteria and viruses.

Scientists at the University of Copenhagen have discovered that Vitamin D is crucial to activating our immune defences and that without sufficient intake of the vitamin, the killer cells of the immune system – T cells – will not be able to react to and fight off serious infections in the body.

For T cells to detect and kill foreign pathogens such as clumps of bacteria or viruses, the cells must first be ‘triggered’ into action and "transform" from inactive and harmless immune cells into killer cells that are primed to seek out and destroy all traces of invaders.

The researchers found that the T cells rely on vitamin D in order activate and they would remain dormant, ‘naïve’ to the possibility of threat if vitamin D is lacking in the blood.

Professor Carsten Geisler from the Department of International Health, Immunology and Microbiology, said: "When a T cell is exposed to a foreign pathogen, it extends a signalling device or ‘antenna’ known as a vitamin D receptor, with which it searches for vitamin D.

"This means that the T cell must have vitamin D or activation of the cell will cease. If the T cells cannot find enough vitamin D in the blood, they won’t even begin to mobilise. ”

The discovery, the scientists believe, provides much needed information about the immune system and will help them regulate the immune response.

This is important not only in fighting disease but also in dealing with anti-immune reactions of the body and the rejection of transplanted organs.

Active T cells multiply at an explosive rate and can create an inflammatory environment with serious consequences for the body.

After organ transplants, T cells can attack the donor organ as a ‘foreign invader’. In autoimmune diseases, like arthritis or Crohns Disease, T cells mistake fragments of the body’s own cells for foreign invaders, leading to the body launching an attack upon itself.

For the research team, identifying the role of vitamin D in the activation of T cells has been a major breakthrough.

“Scientists have known for a long time that vitamin D is important for calcium absorption and the vitamin has also been implicated in diseases such as cancer and multiple sclerosis, but what we didn’t realise is how crucial vitamin D is for actually activating the immune system – which we know now, ” said the researchers.

The findings, continues Professor Geisler, “could help us to contain infectious diseases and global epidemics.

They will be of particular use when developing new vaccines, which work precisely on the basis of both training our immune systems to react and suppressing the body’s natural defences in situations where this is important – as is the case with organ transplants and autoimmune disease.”

Most Vitamin D is produced as a natural by-product of the skin’s exposure to sunlight. It can also be found in fish liver oil, eggs and fatty fish such as salmon, herring and mackerel or taken as a dietary supplement.

The findings are published in the latest edition of Nature Immunology.