We all know getting a few rays of sunshine can be good for you, but we didn't know that the vitamin D synthesized by the body is actually crucial to your thymus gland -- and your immune system. Infection fighting T-cells (which mature in the thymus) need plenty of vitamin D from the bloodstream to do their job, according to a paper published in Nature Immunology (a magazine which, not coincidentally, features the thymus gland quite a bit). "If the T cells cannot find enough vitamin D in the blood, they won't even begin to mobilize," says University of Copenhagen prof Carsten Geisler told Cordis News. So get outside and catch a few rays, why don't ya? Doctor's orders.