Your immune system is not one thing. You have your innate immune system, which includes your primary defenses to bodily insults and injury such as coagulation and inflammation. Then there is the adaptive immune system, which is a large, interwoven, almost indescribably complex framework of different cells and tissues that respond to specific threats in a myriad of ways. Many of them are dependent on each other, but some are more removed from others.

In practical terms, a strong immune system is the result of good self care, especially adequate restful sleep. Your body replenishes its stock of circulating immune cells during sleep. Sleep deprivation induces a state of temporary immunodeficiency, which is why people who don't sleep much or sleep poorly will get sick more often and stay sick longer. It does not matter how healthy you eat or how much you exercise - sleep is the foundation of good health and without enough of it your health will crumble.