Vitamin D3 is a popular supplement because it plays an important role in many different processes within the body. Vitamin D3 isn't just an essential nutrient for healthy bones. It is also known for helping to support healthy immunity as well. Without it, important immune cells such as T-lymphocyte (T-cells) would not develop fully or function properly. While our bodies do produce Vitamin D, many people choose to supplement with this important nutrient because it plays such a vital role in supporting optimal health and wellness.