A lot of interesting research points to an important role for vitamin D (more specifically, calcitriol) in the body's immune system. Most of this research has been done in cultured cells (cells isolated from the body and grown in special solutions of nutrients) and in animals with either severe vitamin D deficiency or whose genes have been altered to “knock out” proteins that control vitamin D metabolism or active vitamin D action.
These types of studies are essential proof of principle that vitamin D is important for the immune system. On top of this, there are a lot of studies that draw associations between either estimated UV light exposure (for example, season or latitude) or serum 25-hydroxyitamin D and certain types of infections or conditions.
Because of these studies, scientists are pretty sure that vitamin D (calcitriol) is important for strengthening the immune system. What isn't known yet is how much vitamin D you need so there is enough calcitriol to maximize the way your immune system works. Research is ongoing.
Certain B cells and T cells have vitamin D receptors and can respond to calcitriol whereas some phagocytes can convert 25-hydroxyvitamin D into calcitriol. This suggests that phagocytes may communicate with T and B cells through calcitriol.
Research studies on the cells of the immune system show that when calcitriol is present, it blocks the features of the adaptive immune system that would lead to autoimmunity. Animal studies show that when calcitriol is absent, the cells of the immune system are more likely to attack the healthy cells of the body (autoimmunity).
Vitamin D deficiency isn’t the only or even the primary cause for autoimmunity — it’s just not good for the immune system when vitamin D is low. Unfortunately, it’s not clear yet how much vitamin D you need in your body to lessen the impact of autoimmunity on health.
Vitamin D, as calcitriol, influences the immune system in two ways:
Calcitriol avoids triggering and arming the T cells during autoimmunity. T cells play a major role in autoimmunity. Calcitriol helps diminish that role and blocks the increased production of the specific helper T cells needed for autoimmunity.
Blocking the production of those cells decreases the ability of T cells to recognize the native protein as foreign, so fewer killer T cells are produced. In other words, the presence of adequate levels of vitamin D and calcitriol keeps the T cells from attacking the body’s own tissues.
Calcitriol blocks chemicals that kill native tissue. As it decreases the number of T cells, calcitriol also diminishes the role of B cells in producing chemicals to destroy native tissue. The antibody response to the body’s own tissue is decreased and the reaction is blocked. For example, diminished destruction of the beta cells of the pancreas, the cause of type 1 diabetes, are proposed to result when adequate levels of vitamin D and calcitriol are present.
