The immune system
What is it?
Many cells work together to protect the body. And the cells sometimes work and sometimes dont. The types of blood cells that the immune system uses is white blood cells. The way it works is that the white blood cells eat up invading organisms.
But this defence system can also cause problems. It will attack donated organs or blood from another person, unless carefully matched. It may overreact to harmless invaders such as pollen grains,causing hay fever. In some cases (autoimmune diseases), the immune system turns against the body's own cells
How effective is it?
As long as your immune system is running smoothly, you don’t notice that it’s there. But if it stops working properly – because it’s weak or can't fight particularly aggressive germs – you get ill. Germs that your body has never encountered before are also likely to make you ill. Some germs will only make you ill the first time you come into contact with them. These include childhood diseases like chickenpox.