"Immune System"

Definition:

The immune system is the collective name for the organs, tissues, and cells of the body that work to prevent and heal disease. Parts of the immune system include the skin and mucous membranes (which prevent germs and toxins from entering the body in the first place); the white blood cells; the spleen and bone marrow; and antibodies.

Examples:

In an allergy, the immune system recognizes a normally harmless substance as dangerous and attacks it.