LONDON (Reuters) – Vitamin D is vital in activating  human defences and low levels suffered by around half the  world’s population may mean their immune systems’ killer T cells  are poor at fighting infection, scientists said yesterday.

The findings by Danish researchers could help the fight  against infectious diseases and global epidemics, they said, and  could be particularly useful in the search for new vaccines.

The researchers found that immune systems’ killer cells,  known as T cells, rely on vitamin D to become active and remain  dormant and unaware of the possibility of threat from an  infection or pathogen if vitamin D is lacking in the blood.

“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,” said Carsten Geisler of  Copenhagen University’s department of international health,  immunology and microbiology, who led the study.

“This means 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.”

Scientists have known for a long time that vitamin D is  important for calcium absorption, and that there is a link  between levels of the vitamin and diseases such as cancer and  multiple sclerosis.

“What we didn’t realise is how crucial vitamin D is for  actually activating the immune system — which we know now,”  Geisler wrote in the study in the journal Nature Immunology.

Most Vitamin D is made by the body 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 supplement.

Almost half of the world’s population has lower than optimal  levels of vitamin D and scientists say the problem is getting  worse as people spend more time indoors.

Geisler and his research team said the findings offered much  needed information about the immune system and would be of  particular use when developing new vaccines.

“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,” they wrote.

Active T cells multiply at an explosive rate and as well as  fighting infection, can also mistakenly attack the body itself.

After and an organ transplant, for example, T cells can  attack the new organ as a “foreign invader”, and in autoimmune  disease, hypersensitive T cells mistake parts of the body’s own  cells as threats, prompting the body to attack itself.

Geisler said there were no definitive studies on the optimal  daily vitamin D dose but experts recommend 25 to 50 micrograms.