When an organism is threatened by microorganisms, viruses, or cancer cells, the immune response acts to provide protection.
Normally, the immune system does not mount a response against self. This lack of an immune response is called tolerance.
In some cases, the immune system does mount an immune response against self. If an error is made, and an immune response is made against self, tolerance to self is lost. This condition is called autoimmunity (from Greek, "self-immunity"). Examples of autoimmune diseases in humans are: asthma, lupus, and arthritis.