The immune system, that complex network of cells and organs that protect us from invasion by the microbes that surround us, is really two systems, both of which can be strengthened by proper nutrition.
Our body's first line of defence against intruders, know as the innate immune system, includes skin, bodily enzymes (such as those in saliva and stomach acid) and epithelial cells (which line, our body's mucosal surfaces).
The second line of defence, known as the adaptive immune system, deals with specific fragments of any potentially harmful or foreign material.
These fragments are called antigens and they are dealt with in the immune system by antigen receptors. Once an antigen receptor adapts to an invading fragment, it retains antigenic memory so that it may quickly neutralise that antigen if it again invades the body.
Both the innate and the adaptive immune systems can be strengthened by certain nutrients, including D vitamins, N-acetyl cysteine, zinc and beta-glucan.