immune system is a collection of organs, special cells, and substances that help protect you from infections and some other diseases. Immune cells and the substances they make travel through your body to protect it from germs that cause infections. They also help protect you from cancer in some ways.

It may help to think of your body as a castle. Germs like viruses, bacteria, and parasites are like hostile, foreign armies that are not normally found in your body. They try to invade your body to use its resources, and they can hurt you in the process. Your immune system is your body’s defense force. It helps keep invading germs out, or kills them if they do get into your body.

The immune system keeps track of all of the substances normally found in the body. Any new substance in the body that the immune system doesn’t recognize raises an alarm, causing the immune system to attack it. Substances that cause an immune response are called antigens. The immune response can destroy anything containing the antigen, such as germs or cancer cells.