Your immune system fights infections and illnesses. It prevents you from getting sick, or, if you do, it helps you get better. It's called the immune system because it's not just one body part. It's a system of different organs, cells, and proteins known as antibodies. Together, they identify, attack, and destroy germs and other foreign substances.
But sometimes the immune system makes a mistake and attacks part of the body. This is called autoimmunity. One example of an autoimmune disease is type 1 diabetes, in which the immune system destroys the cells in the pancreas that produce insulin.