The body's defenses against the microorganisms that cause disease.

The immune system provides the human body with protection from the microorganisms that cause disease. Traditionally scientists viewed the immune system as a defensive network that protected the "self from infectious "non-self invaders. In the mid-1990s, some immunologists modified this view of the immune system, creating a new model of the body's immune system that is able to discriminate between beneficial "non-self invaders (food or helpful bacteria) and threatening invaders. One of the leading scientists investigating the functioning of the immune system in the 1990s was Polly Matzinger of the National Institute of Allergy and Infectious Diseases in Bethesda, Maryland. Matzinger proposed a model of the immune system that responds to invaders only when cells of the body are injured or damaged.

No matter what model is used, immunologists generally agree that the immune systems consists...