Can our bodies attack themselves?

Sometimes the immune system mistakes some harmless material for an invader and this can cause illness.

Grass pollen and dust are harmless materials are often ihaled. In some people the body mounts a fierce attack on them. The immune system releases the substances normally designed to fight infection, and their effects can cause illnesses such as hay fever and asthma. Sometimes the immune system attacks normal tissue or organs, causing a condition called auto-immune disease. This can produce serious illness, but fortunately it is quite rare.