Vitamin A & D are essential nutrients. Vitamin A helps regulate the immune system and assists in many other functions such as eyesight.?Vitamin D helps maintain healthy bones in adults and is essential to Calcium absorption. Helps maintain the health of eyes. Supports the immune system. Helps maintain the skin and health of hair. Plays a role in maintaining bone health. Vitamin A contributes to immune health by supporting the respiratory & gastrointestinal tract, while Vitamin D helps to regulate the function of various immune cells.