Vitamin D deficiency has been linked to a rapidly expanding inventory of ailments—including heart disease, cancer and the common cold. A new discovery demonstrates how the vitamin plays a major role in keeping the body healthy in the first place, by allowing the immune system's T cells to start doing their jobs. 

In order for T cells to become active members of the body's immune system, they must transition from so-called "naive" T cells into either killer cells or helper cells (which are charged with "remembering" specific invaders). And, if ample vitamin D is not around, the T cells do not make that crucial transition, a group of researchers led by Carsten Geisler, head of the Department of International Health, Immunology and Microbiology at the University of Copenhagen, found. They draw this conclusion based on their experiments with isolated naïve human T cells.

"When a T cell is exposed to a foreign pathogen, it extends a signaling device of 'antenna' known as a vitamin D receptor, with which it searches for vitamin D," Geisler said in a prepared statement. If there is an inadequate vitamin D level, he noted, "they won't even begin to mobilize."

Although this vitamin requirement might seem like a handicap to the immune system, the researchers proposed that the additional step involving the vitamin D receptor might actually serve an important evolutionary function: keeping T cells from ravaging healthy tissue. "Given that T cells are capable of explosive proliferation, the lag phase imposed by the vitamin D [receptor step] may diminish the risk of unwanted immunopathology," they noted in the study, which was published online March 7 in Nature Immunology (Scientific American is part of Nature Publishing Group).

The body naturally makes vitamin D when the skin is exposed to sunshine (it can also come from eggs and some fish products), but most people in the U.S. are considered to be deficient in the vitamin. In fact, a 2009 Archives of Internal Medicine study found that 77 percent of U.S. adults and teenagers surveyed did not have the estimated minimum healthful level of 30 nanograms per milliliter in their blood. And just three percent of blacks in the survey were getting enough of the vitamin, the 2009 report found.

The new observation of the vitamin's role in T cell activation could have many implications, including vaccine development (in helping the body to recognize new pathogens) and organ transplant (by discouraging the immune system from attacking a new organ), Geisler noted. Additionally, he added, it "could help us to combat infectious diseases and global epidemics."

Image of a T cell (outlined in red on left) "scanning" another cell to look for invaders courtesy of Carsten Geisler