Overview of the immune system

[Jessica]

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

Microorganisms (commonly called germs, such as bacteria, viruses, and fungi)
Parasites (such as worms)
Cancer cells
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)
What does not (nonself or foreign)

Antigens are any substances that the immune system can recognize and that can thus stimulate an immune response. If antigens are perceived as dangerous (for example, if they can cause disease), they can stimulate an immune response in the body. Antigens may be contained within or on bacteria, viruses, other microorganisms, parasites, or cancer cells. Antigens may also exist on their own—for example, as food molecules or pollen.
A normal immune response consists of the following:

Recognizing a potentially harmful foreign antigen
Activating and mobilizing forces to defend against it
Attacking it
Controlling and ending the attack

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, Hashimoto thyroiditis, or systemic lupus erythematosus (lupus).
Disorders of the immune system occur when

The body generates an immune response against itself (an autoimmune disorder).
The body cannot generate appropriate immune responses against invading microorganisms (an immunodeficiency disorder).
The body generates an excessive immune response to often harmless foreign antigens and damages normal tissues (an allergic reaction).