Immune disorders affect the immune system, a complex system that defends the body against foreign or dangerous invaders. Such invaders include microorganisms (commonly called germs, such as bacteria and viruses), parasites, cancer cells, and even transplanted organs and tissues. To defend the body against these invaders, the immune system must be able to distinguish between what belongs in the body (self) and what does not (nonself or foreign). Any substances that are identified as nonself 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).