The immune system has a vital role: It protects your body from harmful substances, germs and cell changes that could make you ill. It is made up of various organs, cells and proteins. As long as your immune system is running smoothly, you don't notice that it's there.

Which is most important to the immune system?

White blood cells are the key players in your immune system. They are made in your bone marrow and are part of the lymphatic system. White blood cells move through blood and tissue throughout your body, looking for foreign invaders (microbes) such as bacteria, viruses, parasites and fungi.

What would happen without immune system?

Your immune system is your body's defense against infections and other harmful invaders. Without it, you would constantly get sick from germs such as bacteria or viruses. Your immune system is made up of special cells, tissues, and organs that work together to protect you.