Vitamin D is vital in activating human defenses 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 say.

The findings by Danish researchers could help the fight against infectious diseases and global epidemics, they say, 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 signaling device or 'antenna' known as a vitamin D receptor, with which it searches for vitamin D," says 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 mobilize."

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 realize is how crucial vitamin D is for actually activating the immune system - which we know now," Geisler writes 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 say 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 write.

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

After 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 says there are no definitive studies on the optimal daily vitamin D dose but experts recommend 25 to 50 micrograms.