Your immune system
Your immune system is your body's defence against infection and illness. It recognises the cells that make up your body, and will try to get rid of anything unfamiliar. It destroys germs (bacteria and viruses) and parasites. 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 hayfever. In some cases (autoimmune diseases), the immune system turns against the body's own cells.