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Overview of the Immune System

By Peter J. Delves, PhD

The immune system is designed to defend the body against foreign or dangerous invaders. Such invaders include

To defend the body against these invaders, the immune system must be able to distinguish between

  • What belongs in the body (self)

  • What does not (nonself or foreign)

Any substances that are identified as nonself, particularly if they are perceived as dangerous (for example, if they can cause disease), stimulate an immune response in the body. Such substances are called antigens.

Antigens may be contained within or on bacteria, viruses, other microorganisms, or cancer cells. Antigens may also exist on their own—for example, as food molecules or pollen. A normal immune response consists of recognizing a potentially harmful foreign antigen, activating and mobilizing forces to defend against it, and attacking it. If the immune system malfunctions and mistakes self for nonself, it may attack the body’s own tissues, causing an autoimmune disorder, such as rheumatoid arthritis, thyroiditis, or systemic lupus erythematosus (lupus).

Disorders of the immune system occur when

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