Vitamin D
Vitamin D, known as the sunshine vitamin is one of the most important and powerful nutrients for strengthening the immune system. This vitamin has many good sources including fish (salmon, mackerel, tuna, and sardines), dairy products (milk), fruits (oranges), and cereals.
Asides from the direct benefits of this vitamin on the immune system, it helps regulate the amount of calcium and phosphate in the body. Lack of it can lead to bone and teeth deformities in both children and adults.