Vitamin D3 is needed for many bodily functions and to boost the immune system. Vitamin D3 may affect as many as 2,000 genes in the body. Vitamin D has several important functions. Perhaps the most vital are regulating the absorption of calcium and phosphorous and facilitating normal immune system function. Vitamin K and vitamin D work together to increase Matrix GLA Protein (or MGP), the protein responsible for protecting your blood vessels from calcification.