Researchers at the University of Edinburgh say their study has shed light on how vitamin D deficiency may influence the risk of autoimmune diseases.
Vitamin D can "dramatically" affect the immune system and could make people less susceptible to diseases such as multiple sclerosis, scientists have discovered.
The body produces vitamin D in response to sunlight and the scientists investigated how it affects a mechanism in the immune system - dendritic cells' ability to activate T cells.
In healthy people, T cells play a crucial role in helping fight infection, but in people with autoimmune diseases, they can start to attack the body's own tissues.
By studying cells from both mice and humans, researchers found vitamin D caused dendritic cells to produce more of a molecule called CD31 on their surface, and that this hindered the activation of T cells.
CD31 was seen to prevent the two cell types from making a stable contact - an essential part of the activation process - and the resulting immune reaction was far reduced.
Professor Richard Mellanby, of the University of Edinburgh's Centre for Inflammation Research, said: "Low vitamin D status has long been implicated as a significant risk factor for the development of several autoimmune diseases.
"Our study reveals one way in which vitamin D metabolites can dramatically influence the immune system."
The study, published in Frontiers of Immunology, was funded by the Medical Research Council, Biotechnology and Biological Sciences Research Council, Natural Environment Research Council and Wellcome.