According to the research, 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 a foreign pathogen.
Scientists at the University of Copenhagen discovered that the T cells rely on vitamin D in order to activate and they would remain dormant, 'naive' to the possibility of threat if vitamin D is lacking in the blood.
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 realize is how crucial vitamin D is for actually activating the immune system - which we know now," the authors said.
The study has been published in the latest edition of Nature Immunology.